Probe incorporation mediated by enzymes

ABSTRACT

The invention provides compositions and methods of use thereof for labeling peptide and proteins in vitro or in vivo. The methods described herein employ lipoic acid ligase or mutants thereof, and lipoic acid analogs recognized by lipoic acid ligase and lipoic acid ligase mutants.

GOVERNMENT SUPPORT

This invention was made with government support awarded by the NationalInstitutes of Health under Grant Number R01 GM072670-01. The Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

Biophysical probes such as fluorophores, spin labels, and photoaffinitytags have greatly improved the understanding of protein structure andfunction in vitro, and there is great interest in using them insidecells to study proteins within their native context. The majorbottleneck to using such probes inside cells, however, is the difficultyof targeting the probes with very high specificity to particularproteins of interest, given the chemical heterogeneity of the cellinterior. The most prominent method for labeling cellular proteins is togenetically encode green fluorescent protein (GFP) or one of itsvariants as a fusion to the protein of interest. Because GFPs aregenetically encoded, their labeling is absolutely specific and GFPvariants have proven extremely useful for in vivo studies of proteinlocalization, however, they still have severe limitations such as theirlarge size (˜235 amino acids), which can perturb the function of theprotein of interest, and the fact that they are not very bright and onlyamenable to optical microscopy. For example, the best of the previouslydescribed methods, the FlAsH labeling method uses an extremely smalltetracysteine motif to direct a biarsenical-containing probe. Thismethod has yielded exciting new biological information, but suffers frompoor specificity, and cell toxicity. Most other methods such as theSNAP/AGT, Halotag, DHFR, FKBP(12), and single-chain antibody methods useprotein rather than peptide-based targeting sequences, raising concernsabout steric interference with receptor function. Peptide-basedtargeting methods include FlAsH, His₆-tag labeling, phosphopantetheinyltransferase labeling, transglutaminase labeling, and keto/biotin ligaselabeling. His₆ labeling and FlAsH suffer from probe dissociation,whereas ketone/biotin lipase and transglutaminase are restricted tolabeling at the cell surface.

SUMMARY OF THE INVENTION

The invention relates in part to labeling of proteins (or fragmentsthereof) using lipoic acid ligase and/or lipoic acid ligase mutants.Methods and compositions of the invention provide labeling specificitywhile also expanding the scope of compatible probe structures forlabeling of proteins. Labeling of polypeptides or proteins can beperformed in vitro or in vivo. The invention also provides, inter alia,lipoic acid ligase mutants, lipoic acid analogs, and acceptorpolypeptides and methods of use thereof for labeling proteins. It alsoprovides screening methods for identifying further lipoic acid ligasemutants, lipoic acid analogs, and acceptor polypeptides.

According to one aspect of the invention, methods for labeling a targetprotein are provided. the methods include contacting a fusion proteinwith a lipoic acid analog, and allowing sufficient time for the lipoicacid analog to be conjugated to the fusion protein via an acceptorpolypeptide, in the presence of a lipoic acid ligase or mutant thereof,wherein the fusion protein is a fusion of the target protein and theacceptor polypeptide. In some embodiments, the lipoic acid analogcomprises an alkyl azide, or an alkyne carboxylic acid, an aryl azidephotoaffinity probe, or a fluorophore substrate. In certain embodiments,the lipoic acid analog is detectably labeled. In some embodiments, thelipoic acid analog is directly detectable. In some embodiments, thedirectly detectable label is coumarin, fluorescein, an aryl azide, adiazirine, a benzophenone, a resorufin, a xanthene-type fluorophore, achloroalkane, a metal-binding ligand, or a derivative thereof. In someembodiments, the detectable label is coumarin. In certain embodiments,the lipoic acid analog is labeled with an indirectly detectable label.In some embodiments, the indirectly detectable label is an enzyme. Insome embodiments, the lipoic acid analog is labeled with a membraneimpermeant label. In certain embodiments, the lipoic acid analog islabeled after conjugation to the fusion protein. In some embodiments,the lipoic acid analog is labeled with a cyclooctyne conjugate. In someembodiments, the cyclooctyne conjugate is detectably labeled. In certainembodiments, the detectable label is coumarin, fluorescein, a arylazide, a diazirine, a benzophenone, a resorufin, a xanthene-typefluorophore, a chloroalkane, a metal-binding ligand, or a derivativethereof. In some embodiments, the target protein is a cell surfaceprotein. In some embodiments, the fusion protein is in a cell. In someembodiments, the cell expresses the lipoic acid ligase or mutantthereof. In certain embodiments, the cell is a eukaryotic cell. In someembodiments, the cell is a bacterial cell. In certain embodiments, theacceptor polypeptide comprises an amino acid sequence of any one of SEQID NO:1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, or a functional variant thereof.In some embodiments, the functional variant of any one of SEQ ID NO:1,2, 3, 4, 5, 6, 7, 8, 9, or 10 comprises an amino acid sequence that hasup to 85%, 90%, 95%, or 99% identity to SEQ ID NO:1, 2, 3, 4, 5, 6, 7,8, 9, or 10 and is a substrate for a lipoic acid ligase or mutantthereof. In some embodiments, the acceptor polypeptide comprises anamino acid sequence of SEQ ID NO: 10. In some embodiments, the acceptorpolypeptide is N- or C-terminally fused to the target protein. Incertain embodiments, the lipoic acid ligase is an E. coli lipoic acidligase or mutant thereof. In some embodiments, the lipoic acid ligase isLplA. In some embodiments, the lipoic acid ligase mutant includes anamino acid sequence of wild-type LplA that includes a substitution atone or more of residues corresponding to residue 16, 17, 19, 20, 21, 37,37 +71, 37 +20, 37 +35, 35, 41, 70, 71, 72, 79, 85, 87, 140, 147, and149 of SEQ ID NO:11. In certain embodiments, the lipoic acid ligasemutant comprises an amino acid sequence of LplA having one or more ofthe amino acid substitution corresponding to substitution of N16A, L17A,V19A, E20A, E21A, W37A, W37G, W37S, W37V, W37A +S71A, W37A +E20A, W37L,W37I, W37T, W37N, W37V +E20G, W37V +F35A, W37V +E20A, F35A, N41A, R70A,S71A, S72A, H79A, C85A, T87A, R140A, F147A, H149A, or H149V of SEQ IDNO:11. In some embodiments, the lipoic acid ligase comprises the aminoacid sequence set forth as SEQ ID NO:11. In some embodiments, the lipoicacid ligase mutant comprises an amino acid sequence that has up to 85%,90%, 95%, or 99% identity to the amino acid sequence of SEQ ID NO:11 andligates lipoic acid and/or a lipoic acid analog to an acceptorpolypeptide. In certain embodiments, the lipoic acid ligase is a homologof an E. coli lipoic acid ligase or a mutant of a homolog of an E. colilipoic acid ligase. In some embodiments, the lipoic acid ligase isThermoplasma acidophilum LplA; Plasmodium falciparum LipL1, or LipL2;Oryza Sativa LplA; Streptococcus pneumoniae LplA; or a homolog fromPyrococcus horikoshii; Saccharomyces cerevisiae; Trypanosoma cruzi;Bacillus subtilis; or Leuconostoc mesenteroides. In some embodiments,the method is performed in a cell-free environment. In some embodiments,the method is performed in a cell. In certain embodiments, the acceptorpolypeptide is fused to the target protein via a cleavable bond orlinker.

According to another aspect of the invention, composition that include alipoic acid ligase mutant that binds to a lipoic acid analog areprovided. In some embodiments, the lipoic acid ligase mutant comprisesan amino acid substitution in a lipoic acid interaction and activationdomain. In some embodiments, the lipoic acid ligase mutant comprises anamino acid sequence of wild-type LplA comprising a substitution at oneor more of residues 16, 17, 19, 20, 21, 37, 37 +71, 37 +20, 37 +35, 35,41, 70, 71, 72, 79, 85, 87, 140, 147, and 149. In certain embodiments,the lipoic acid ligase mutant comprises the amino acid sequence of LplAhaving one or more of the amino acid substitutions corresponding to asubstitution N16A, L17A, V19A, E20A, E21A, W37A, W37G, W37S, W37V, W37A+S71A, W37A +E20A, W37L, W37I, W37T, W37N, W37V +E20G, W37V +F35A, W37V+E20A, F35A, N41A, R70A, S71A, S72A, H79A, C85A, T87A, R140A, F147A,H149A, or H149V of SEQ ID NO:11. In some embodiments, the lipoic acidligase mutant comprises an amino acid sequence that has up to 85%, 90%,95%, or 99% identity to a wild-type lipoic acid ligase sequence andligates lipoic acid and/or a lipoic acid analog to an acceptorpolypeptide. In some embodiments, the wild-type lipoic acid ligasesequence is the sequence set forth as SEQ ID NO:11. In some embodiments,the lipoic acid ligase homolog or a mutant of a homolog of an E. colilipoic acid ligase. In certain embodiments, the lipoic acid ligase isThermoplasma acidophilum LplA; Plasmodium falciparum LipL1, or LipL2;Oryza Sativa LplA; Streptococcus pneumoniae LplA; or a homolog fromPyrococcus horikoshii; Saccharomyces cerevisiae; Trypanosoma cruzi;Bacillus subtilis; or Leuconostoc mesenteroides. In some embodiments,the lipoic acid ligase mutant isolated. In some embodiments, the lipoicacid ligase mutant has altered binding affinity to lipoic acid comparedto wild-type lipoic acid ligase. In certain embodiments, the lipoic acidligase mutant has wild type binding affinity to lipoic acid. In someembodiments, the lipoic acid analog is an alkyl azide, an alkynecarboxylic acid, an aryl azide photoaffinity probe, or a fluorophoresubstrate. In some embodiments, the alkyl azide is a modified alkylazide and the alkyne carboxylic acid is a modified alkyne carboxylicacid. In some embodiments, the lipoic acid analog comprises coumarin.

According to yet another aspect of the invention, composition thatinclude a nucleic acid encoding a lipoic acid ligase mutant that bindsto a lipoic acid analog are provided. In certain embodiments, thenucleic acid sequence comprises the nucleotide sequence set forth as SEQID NO:12. In some embodiments, the lipoic acid ligase mutant has anucleic acid sequence that has up to 85%, 90%, 95%, or 99% identity tothe nucleic acid sequence of a wild-type lipoic acid ligase and ligateslipoic acid and/or a lipoic acid analog to an acceptor polypeptide. Insome embodiments, the nucleic acid of the wild-type lipoic acid ligasehas the nucleotide sequence set forth as SEQ ID NO:12. In certainembodiments, the lipoic acid ligase mutant comprises an amino acidsequence of wild-type LplA comprising a substitution at one or more ofresidues corresponding to residue 16, 17, 19, 20, 21, 37, 37 +71, 37+20, 37 +35, 35, 41, 70, 71, 72, 79, 85, 87, 140, 147, and 149 of setSEQ ID NO:11. In some embodiments, the lipoic acid ligase mutantcomprises the amino acid sequence of LplA with one or more of the aminoacid substitutions corresponding to a substitution of N16A, L17A, V19A,E20A, E21A, W37A, W37G, W37S, W37V, W37A +S71A, W37A +E20A, W37L, W37I,W37T, W37N, W37V +E20G, W37V +F35A, W37V +E20A, F35A, N41A, R70A, S71A,S72A, H79A, C85A, T87A, R140A, F147A, H149A, or H149V of SEQ ID NO:11.In some embodiments, the nucleic acid is isolated. In certainembodiments,

According to another aspect of the invention, a vector that includes anyof the aforementioned nucleic acids of any forgoing aspect of theinvention are provided.

According to another aspect of the invention, a host cell that includesany of the aforementioned vectors of any forgoing aspect of theinvention are provided. In some embodiments, the nucleic acid isinducibly expressed.

According to yet another aspect of the invention, a process forpreparing a lipoic acid ligase mutant is provided. The process includesculturing any aforementioned host cell of any foregoing aspect of theinvention and recovering the lipoic acid ligase mutant from the culture.

According to another aspect of the invention, compositions are provided.The compositions include a lipoic acid analog that binds to lipoic acidligase and/or a mutant thereof, wherein the lipoic acid analog is amodified alkyl azide or a modified alkyne carboxylic acid. In someembodiments, the lipoic acid analog is an alkyl azide, a linear alkyne,or an alkyl halide. In some embodiments, the lipoic acid analog isisolated.

According to another aspect of the invention, a composition is provided.The composition includes a lipoic acid analog that binds to lipoic acidligase or mutant thereof, wherein the lipoic acid analog is an arylazide, diazirine, or benzophenone photoaffinity probe or a fluorophoresubstrate. In certain embodiments, the lipoic acid analog is a4-azido-2,3,5,6-tetrafluorobenzoic derivative, a 7,7′-azo-octanoic acid,a benzophenone, or a 6,8-difluoro-7-hydroxycoumarin fluorophorederivative. In some embodiments, the lipoic acid analog is isolated.

According to yet another aspect of the invention, compositions areprovided. The compositions include an acceptor polypeptide thatfunctions as a substrate for a lipoic acid ligase or mutant thereof andcomprises an amino acid sequence of SEQ ID NO:1, 2, 3, 4, 5, 6, 7, 8, 9,or 10, or a functional variant thereof. In some embodiments, theacceptor polypeptide functional variant comprises an amino acid sequencethat has up to 85%, 90%, 95%, or 99% identity to at least one of SEQ IDNOs:1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 and is a substrate for a lipoicacid ligase or mutant thereof. In some embodiments, the acceptorpolypeptide comprises an amino acid sequence of SEQ ID NO: 10. Incertain embodiments, the acceptor polypeptide is N- or C-terminallyfused to a target protein.

According to yet another aspect of the invention, methods foridentifying a lipoic acid ligase having specificity for a lipoic acidanalog are provided. The methods include contacting a lipoic acid orlipoic acid analog with an acceptor polypeptide in the presence of acandidate lipoic acid ligase molecule, and detecting a lipoic acid orlipoic acid analog that is bound to the acceptor polypeptide, whereinthe presence of a lipoic acid or lipoic acid analog bound to an acceptorpolypeptide indicates that the candidate lipoic acid ligase molecule isa lipoic acid ligase that has specificity for the lipoic acid or lipoicacid analog. In some embodiments, the lipoic acid ligase is a homolog ormutant lipoic acid ligase. In some embodiments, the lipoic acid orlipoic acid analog is directly detectable. In some embodiments, thelipoic acid analog is coumarin. In certain embodiments, the lipoic acidor lipoic acid analog is conjugated to a detectable label. In someembodiments, the detectable label is a directly detectable label. Insome embodiments, the directly detectable label is a fluorophore. Incertain embodiments, the detectable label is an indirectly detectablelabel. In some embodiments, the indirectly detectable label is anenzyme. In some embodiments, detecting a lipoic acid or lipoic acidanalog comprises detecting the detectable label conjugated to the lipoicacid or lipoic acid analog. In certain embodiments, the acceptorpolypeptide comprises an amino acid sequence of one of SEQ ID NO:1, 2,3, 4, 5, 6, 7, 8, 9, or 10 or a functional variant thereof. In someembodiments, the functional variant comprises an amino acid sequencethat has up to 85%, 90%, 95%, or 99% identity to SEQ ID NO:10 and is asubstrate for a lipoic acid ligase or mutant thereof. In someembodiments, the acceptor polypeptide has an amino acid sequencecomprising SEQ ID NO:10. In some embodiments, the lipoic acid analog isan alkyl azide, an alkyne carboxylic acid, an aryl azide affinity probe,or a fluorophore substrate. In certain embodiments, the lipoic acidanalog is detected using a fluorescent detection system, a luminescentdetection system, an enzyme detection system, or an optical detectionsystem. In some embodiments, the method also includes removing unboundlipoic acid or lipoic acid analog prior to detecting bound lipoic acidanalog. In some embodiments, the method also includes isolating thecandidate molecule that is a lipoic acid ligase mutant havingspecificity for the lipoic acid or lipoic acid analog.

According to yet another aspect of the invention, methods foridentifying a lipoic acid analog having specificity for a lipoic acidligase or a mutant thereof, are provided. The methods include combiningan acceptor polypeptide with a candidate lipoic acid analog molecule inthe presence of a lipoic acid ligase or mutant thereof and determiningthe presence of lipoic acid analog incorporation, wherein lipoic acidanalog incorporation is indicative of a candidate lipoic acid analoghaving specificity for a lipoic acid ligase or mutant thereof. In someembodiments, the lipoic acid analog comprises an alkyl azide, an alkynecarboxylic acid, a modified alkyl azide, a modified alkyne carboxylicacid, an aryl azide affinity probe, a diazirine affinity probe, abenzophenone affinity probe, or a fluorophore substrate. In certainembodiments, the acceptor polypeptide comprises an amino acid sequenceof SEQ ID NO:1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, or a variant thereof. Insome embodiments, the variant of the acceptor polypeptide comprises anamino acid sequence that has up to 85%, 90%, 95%, or 99% identity to anyone of the amino acid sequences set forth as SEQ ID NO:1, 2, 3, 4, 5, 6,7, 8, 9, or 10 and is a substrate for a lipoic acid ligase or mutantthereof. In some embodiments, the acceptor polypeptide has an amino acidsequence comprising SEQ ID NO:10. In some embodiments, the lipoic acidanalog is directly detectable. In certain embodiments, the lipoic acidanalog is coumarin. In some embodiments, the lipoic acid analog isconjugated to a detectable label. In some embodiments, the detectablelabel is a directly detectable label. In some embodiments, the directlydetectable label is a fluorophore. In certain embodiments, thedetectable label is an indirectly detectable label. In some embodiments,the indirectly detectable label is an enzyme. In some embodiments,detecting a lipoic acid analog comprises detecting the detectable labelconjugated to the lipoic acid analog. In certain embodiments, the lipoicacid analog is detected using a fluorescent detection system, aluminescent detection system, an enzyme detection system, or an opticaldetection system. In some embodiments, the method also includes removingunbound lipoic acid analog prior to detecting bound lipoic acid analog.In some embodiments, the method also includes isolating the candidatemolecule that is a lipoic acid ligase mutant having specificity for alipoic acid analog.

According to another aspect of the invention, methods for identifying anacceptor polypeptide having specificity for a lipoic acid ligase ormutant thereof are provided. The methods include combining an candidateacceptor polypeptide with a labeled lipoic acid or analog thereof in thepresence of a lipoic acid ligase or mutant thereof and determining alevel of lipoic acid or lipoic acid analog incorporation, wherein lipoicacid or lipoic acid analog incorporation is indicative of a candidateacceptor polypeptide having specificity for a lipoic acid ligase ormutant thereof. In some embodiments, the lipoic acid analog comprises analkyl azide, an alkyne carboxylic acid, a modified alkyl azide, amodified alkyne carboxylic acid, an aryl azide affinity probe, adiazirine affinity probe, a benzophenone, or a fluorophore substrate. Incertain embodiments, the acceptor polypeptide comprises a variant of anamino acid sequence of SEQ ID NO:1, 2, 3, 4, 5, 6, 7, 8, 9, or 10. Insome embodiments, the variant of the acceptor polypeptide comprises anamino acid sequence that has up to 85%, 90%, 95%, or 99% identity to anyone of the amino acid sequences set forth as SEQ ID NO:1, 2, 3, 4, 5, 6,7, 8, 9, or 10 and is a substrate for a lipoic acid ligase or mutantthereof. In some embodiments, the lipoic acid analog is directlydetectable. In some embodiments, the lipoic acid analog is coumarin. Incertain embodiments, the lipoic acid analog is conjugated to adetectable label. In some embodiments, the detectable label is adirectly detectable label. In some embodiments, the directly detectablelabel is a fluorophore. In some embodiments, the detectable label is anindirectly detectable label. In certain embodiments, the indirectlydetectable label is an enzyme. In some embodiments, detecting a lipoicacid analog comprises detecting the detectable label conjugated to thelipoic acid analog. In some embodiments, the lipoic acid analog isdetected using a fluorescent detection system, a luminescent detectionsystem, an enzyme detection system, or an optical detection system. Insome embodiments, the method also includes removing unbound lipoic acidanalog prior to detecting bound lipoic acid analog. In certainembodiments, the method also includes isolating the candidate moleculethat is a lipoic acid ligase mutant having specificity for a lipoic acidanalog.

These and other objects of the invention will be described in furtherdetail in connection with the detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram illustrating synthetic routes to alkyl azide andalkyne probes and for aryl azide and coumarin derivatives. Note that n=4and n=8 alkynes were purchased from TCI America.

FIG. 2 shows a mass spectrometric trace and a Michaelis-Menten plotshowing characterization of LplA-catalyzed azide 7 ligation. FIG. 2Ashows a mass spectrometric analysis of E2p-azide 7 conjugate (starredproduct in HPLC trace shown in FIG. 5). The spectrum for E2p is shownfor reference. Peaks for the +6, +7, +8, and +9 charge states areobserved. FIG. 2B is a Michaelis-Menten plot for azide 7 ligation toE2p. Initial rates are shown as a function of azide 7 concentration.LplA concentration was 200 nM. The measured k_(cat) was 0.111±0.003 s⁻¹.Each data point represents the average of three independent experiments.Error bars, ±1 s.d.

FIG. 3 shows peptides prepared for the engineering of a peptidesubstrate for LplA.

GLNDIFEADKAEWHE is SEQ ID NO: 1; GDTLCIVEADKAMNQIE is SEQ ID NO: 2;GDTLCIVEADKASMEIP is SEQ ID NO: 3; EQSLITVEGDKASMEVP is SEQ ID NO: 4;DDVLCEVQNDKAVVEIP is SEQ ID NO: 5; DEVLVEIETDKVVLEVP is SEQ ID NO: 6;GDDCAVAESVKAASDIY is SEQ ID NO: 7; DEVLVEIETDKAVLEVP is SEQ ID NO: 8;DEVLVEIETDKAVLEVP is SEQ ID NO: 9; andDEVLVEIETDKAVLEVPGGEEE is SEQ ID NO: 10.

FIG. 4 provides diagrams of synthetic routes to cyclooctyne-probeconjugates. FIG. 4A shows OCT acid 1(5) was activated as thepentafluorophenyl (PFP) ester 2, then conjugated to a diaminopolyethylene glycol (PEG) linker to give OCT-PEG 3. Reaction of 3 withthe activated N-hydroxy succinimidyl (NHS) esters of Cy3 or Alexa Fluor568 gave the final fluorophore conjugates 4 and 5 (FIG. 4B). Thesynthesis of OCT-PEG-biotin 6 (FIG. 4B) has been described (5).

FIG. 5 shows reaction diagrams, a substrate conversion table and HPLCassay traces demonstrating re-directing LplA for site-specific proteinlabeling with fluorescent probes. FIG. 5A shows the natural reactioncatalyzed by LplA (top), and scheme for LplA-catalyzed fluorescenttagging with unnatural probes (bottom). Instead of lipoic acid, LplAligates an alkyl azide to a lysine sidechain within a peptiderecognition sequence. The azide is then selectively functionalized witha cyclooctyne-probe conjugate (dark circle), to give a triazole adduct.FIG. 5B shows a comparison of alkyl azide and alkyne substrates of LplA.Conversions are given relative to lipoic acid, which is normalized to100%. FIG. 5C shows results from an HPLC assay showing the ligation ofthe azide 7 substrate to E2p protein. The starred peak was analyzed bymass-spectrometry (see FIG. 2). FIGS. 5 D and 5E show characterizationof W37V LplA mutant-catalyzed aryl azide ligation. FIG. 5D shows an HPLCassay showing the ligation of the aryl azide substrate to LAPHP1. FIG.5E shows mass spectromtetric analysis of LAPHP1-aryl azide conjugate(starred product in HPLC trace). Peaks for the +8, +9, +10, +11, +12,+13, +14, and +15 charge states were observed. FIGS. 5F and 5G showcharacterization of W37V/E20G Lpl mutant-catalyzed coumarin (a pacificblue derivative) ligation. FIG. 5F shows an HPLC assay showing theligation of the coumarin to LAPHP1. FIG. 5G shows mass spectrometricanalysis of E2p-coumarin conjugate (starred product in HPLC trace).Peaks for the +6, +7, +8, and +9 charge states were observed.

FIG. 6 shows western blots demonstrating that LplA labels the LAPpeptide without modifying endogenous mammalian proteins. Lysates fromHEK cells expressing a LAP fusion to CFP were labeled in vitro with LplAand azide 7. The azide was derivatized with phosphine-FLAG via theStaudinger ligation (14), and the FLAG epitope was detected by blottingwith an anti-FLAG antibody. Controls are shown with LAP-CFP replaced byits alanine point mutant (lane 3), or with LplA replaced by itscatalytically inactive Lys133Ala mutant (lane 2). Coomassie stainingdemonstrates equal loading in all lanes. Fluorescence visualization ofCFP demonstrates equal expression levels of the LAP fusion in lanes 1-3.

FIG. 7 shows examples of synthetic structures of substrate analogs thatmay be used by Lpl1 including: an alkyl azide; an alkyne; a halide; a4-azido-2,3,5,6-tetrafluorobenzoic derivative; a 7,7′-azo-octanoic acid;a benzophenone; and a 6,8-difluoro-7-hydroxycoumarin fluorophorederivative.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to protein labeling in vivo and in vitro. Priorattempts to label specific proteins have been frustrated by a lack ofreagents with sufficient specificity. The invention aims to overcomethis lack of specificity through the use of lipoic acid ligase andmutants thereof with lipoic acid analogs and acceptor polypeptides thatare recognized by lipoic acid ligase and mutants thereof. The inventionincludes, in part, use of a lipoic acid ligase to site-specifically andcovalently attach small molecules to proteins modified by a shortpeptide tag.

The invention therefore provides, inter alia, methods for labelingproteins in vitro or in vivo. The method generally involves contacting alipoic acid analog with a fusion protein comprising an acceptorpolypeptide in the presence of a lipoic ligase mutant, and allowingsufficient time for conjugation of the lipoic acid analog to the fusionprotein. Times and reaction conditions suitable for mutant lipoic acidligase activity will generally be comparable to those for wild-typelipoic acid ligase which are known in the art. (See for example,Examples 1 and 2 herein).

The various components of this reaction will be described in greaterdetail herein. Briefly, the fusion protein is a fusion of the targetprotein (i.e., the protein which is to be labeled) and an acceptorpolypeptide (i.e., the peptide sequence that acts as a substrate for thelipoic acid ligase mutant). If the method is performed in vivo, thenucleic acid sequence encoding the fusion protein may be introduced intothe cell and transcription and translation allowed to occur. In someembodiments, the fusion protein may be present in a cell in a subject.In some embodiments, the fusion protein may be present in a transgenicsubject. If the method is performed in vitro, the fusion protein maysimply be added to the reaction mixture.

As used herein, protein labeling in vitro means labeling of a protein ina cell free environment. As an example, such a protein can be combinedwith a lipoic acid ligase mutant and a lipoic acid analog underappropriate conditions and thereby labeled, in for example a test tubeor a well of a multiwell plate.

As used herein, protein labeling in vivo means labeling of a protein inthe context of a cell. The method can be used to label proteins that areintracellular proteins or cell surface proteins. The cell may be presentin a subject (e.g., an insect such as Drosophila, a rodent such as amouse, a human, and the like) or it may be present in culture. In someembodiments, a subject may be a transgenic subject.

A lipoic acid ligase or mutant thereof may also be expressed by the cellin some instances. In other instances, however, the lipoic acid ligaseor mutant thereof may simply be added to the reaction mixture (if invitro) or to the cell (if the target protein is a cell surface proteinand the acceptor peptide is located on the extracellular domain of thetarget protein).

According to the method, the lipoic acid ligase or mutant thereofconjugates the lipoic acid analog to the acceptor polypeptide that isfused (either at the nucleic acid level or post-translationally) to thetarget protein. The method is independent of the protein type and thusany protein can be labeled in this manner. The product of this labelingreaction may or may not be directly detectable however depending uponthe nature of the lipoic acid analog, as described herein. Accordingly,it may be necessary to react the conjugated lipoic acid analog with adetectable label. If the method is performed in vivo, the detectablelabel may be one capable of diffusion into a cell. If the method is usedto label a cell surface protein, then the lipoic acid analog may belabeled with a membrane impermeant label in order to reduce entry andaccumulation of the label intracellularly. The lipoic acid analog may belabeled prior to or after conjugation to the fusion protein.

Labeling of proteins allows one to track the movement and activity ofsuch proteins. It also allows cells expressing such proteins to betracked and imaged, as the case may be. The methods can be used in cellsfrom virtually any organism including insect, yeast, frog, worm, fish,rodent, human and the like.

The method can be used to label virtually any protein. Examples includebut are not limited to signal transduction proteins (e.g., cell surfacereceptors, kinases, adapter proteins), nuclear proteins (transcriptionfactors, histones), mitochondrial proteins (cytochromes, transcriptionfactors) and hormone receptors.

Lipoic acid ligase is an enzyme that catalyzes the ATP-dependentligation of the small molecule lipoic acid to a specific lysinesidechain within one of three natural acceptor proteins E2p, E2o, andH-protein. As used herein, wild-type lipoic acid ligase refers to anaturally occurring E. coli lipoic acid ligase having wild-type lipoicacid ligase activity, or to a homolog thereof. SEQ ID NO:11 representsthe amino acid sequence of E. coli wild-type lipoic acid ligase. Theamino acid sequence of SEQ ID NO:11 is based on the crystal structure ofthe wild-type LplA, which differs from the GenBank sequence set forth asAccession No. AAA21740, because the first amino-acid (methionine) hasbeen cleaved. SEQ ID NO:11 is missing the initial methionine in theamino acid sequence set forth as Accession No. AAA21740 and thereforethe amino acid numbering set forth of wild-type lipoic acid and used toidentify substituted amino acid residues in modified/mutant lipoic acidligases of the invention, differs by one amino acid from the numberingof the amino acids in Accession No. AAA21740. Numbering of amino acidsof wild-type lipoic acid of the invention as used herein corresponds tothe numbering of SEQ ID NO:11. Those of ordinary skill in the art willbe readily able to convert the numbering of amino acids based on SEQ IDNO:11 with those of GenBank Accession No. AAA21740. SEQ ID NO: 12represents the nucleotide sequence of E. coli wild-type lipoic acidligase (GenBank Accession No. L27665).

Lipoic acid ligase is also known as lipoate-protein ligase A, LplA, andlipoate-protein ligase. In some embodiments of the invention, the lipoicacid ligase is an E. coli lipoic acid ligase, such as LplA. Homologs ofE. coli lipoic acid ligase include, but are not limited to: Thermoplasmaacidophilum LplA; Plasmodium falciparum LipL1, or LipL2; Oryza SativaLplA (rice); Streptococcus pneumoniae LplA; and homologs from Pyrococcushorikoshii; Saccharomyces cerevisiae, Trypanosoma cruzi, Bacillussubtilis, and Leuconostoc mesenteroides. Homologs of E. coli lipoic acidligase as well as mutants of such homologs are useful in methods andcompositions of the invention.

The reaction between wild-type lipoic acid ligase and its substrate(discussed below) is referred to as orthogonal. This means that neitherthe ligase nor its substrate react with any other enzyme or moleculewhen present either in their native environment (i.e., a bacterial cell)or more importantly for the purposes of the invention in a non-nativeenvironment (e.g., a mammalian cell). Accordingly, the invention takesadvantage of the high degree of specificity that has evolved betweenwild-type lipoic acid ligase and its substrate. Ligation interactions ofthe invention may or may not be orthogonal ligation reactions, it is notrequired that the ligation reactions of the invention be orthogonal. Theonly known natural substrates in bacteria of wild-type E. coli lipoicacid ligase are E2p, E2o, and H-protein, which are ligated to lipoicacid by the enzyme. The natural reaction of LplA has now been redirectedsuch that unnatural structures, dissimilar to lipoic acid, can beligated to either the natural protein substrates or LplA, or engineeredpeptide substrates.

A 12-17 amino acid minimal substrate sequence encompasses a lysinelipoylation site at the tip of a sharp β-turn in the substrate (e.g.,such as E2o, E2p, or H-protein). For example in E. coli E2o, the lysineat the tip of a sharp β-turn is the lysine that is in position 44 of E.coli E2o, see GenBank Accession No. AAA23898. In each of the threelipoyl domains of E. coli E2p, the lysines at the tip of the sharpβ-turn are the lysine lipoylation sites (e.g., the lysine in position ofthe lipoyl hybrid domain, see ProteinDataBank Accession No. 1QJO). In E.coli H-protein, the lysine at the tip of a sharp β-turn is the lysinethat is in position 65 of E. coli H-protein, see GenBank Accession No.CAA52145. Testing has shown that although accurate positioning of thetarget lysine within the β-turn is important for LplA recognition, theresidues flanking the lysine can be varied.

As used herein, an “acceptor peptide” is a protein or peptide having anamino acid sequence that is a substrate for a lipoic acid ligase, lipoicacid ligase, or mutant thereof, a lipoic acid ligase homolog or mutantthereof (i.e., a lipoic acid ligase homolog or mutant recognizes and iscapable of conjugating a lipoic acid analog or lipoic acid to thepeptide). The acceptor peptide may have an amino acid sequence of Xaa₁Xaa₂ Xaa₃ Xaa₄ Xaa₅ Xaa₆ Xaa₇ Xaa₈ Xaa₉ Lys Xaa₁₀ Xaa₁₁ Xaa₁₂ Xaa₁₃Xaa₁₄, where Xaa₁₋₁₄ is any amino acid that results in the structure ofthe polypeptide suitable for use in methods and compositions of theinvention. In an exemplary 15 amino acid acceptor polypeptide coresequence such as Xaa₁ Xaa₂ Xaa₃ Xaa₄ Xaa₅ Xaa₆ Xaa₇ Xaa₈ Xaa₉ Lys Xaa₁₀Xaa₁₁ Xaa₁₂ Xaa₁₃ Xaa₁₄, amino acids Xaa₇, Xaa₉ and Xaa₁₃ may be highlyconserved among species and in some embodiments they will be amino acidsE, D, and E respectively. In addition, amino acids Xaa₃, Xaa₆, and Xaa₁₄may be relatively conserved and they are hydrophobic amino acids in someembodiments may be I, V or L. In some embodiments, amino acid Xaa₂ maybe one of D, E or Q based on sequence alignment of different species.One of ordinary skill in the art will understand how to interpret thepositions of amino acids in a 15 amino acid core sequence with those ina shorter acceptor polypeptide sequence, for example in a 12, 13, or 14amino acid acceptor polypeptide sequence, based on the position of thelysine residue in the sequences. Although the exemplary sequence Xaa₁Xaa₂ Xaa₃ Xaa₄ Xaa₅ Xaa₆ Xaa₇ Xaa₈ Xaa₉ Lys Xaa₁₀ Xaa₁₁ Xaa₁₂ Xaa₁₃Xaa₁₄ a 15 amino acid polypeptide, longer acceptor polypeptides andshorter acceptor polypeptides are also useful in methods andcompositions of the invention. Non-limiting examples of polypeptidesubstrates useful in the invention are provided in FIG. 3.

In important embodiments, the acceptor peptide comprises the amino acidsequence of a polypeptide having SEQ ID NO:1, 2, 3, 4, 5, 6, 7, 8, 9, or10, set forth in FIG. 3, or may be a variant thereof. A variantpolypeptide may include a portion of an amino acid set forth herein asSEQ ID NOs:1-10, (e.g., may be a 12, 13, or 14 amino acid portion aslong as it includes the lysine residue and functions as an acceptorpolypeptide), or may include the full sequence of one of SEQ ID NO:1-10with additional amino acids attached at one or both ends of thepolypeptide. As long as a polypeptide includes the positioning of thetarget lysine within the β-turn such that the polypeptide functions as asubstrate for lipoic acid enzyme as described herein, (e.g., wild-type,homolog, and/or mutants thereof) the remainder of the polypeptidesequence can vary.

In some embodiments of the invention, an acceptor polypeptide thatfunctions as a substrate for a lipoic acid ligase or mutant thereofincludes an amino acid sequence of SEQ ID NO:1, 2, 3, 4, 5, 6, 7, 8, 9,or 10, or a functional variant thereof. A functional variant of anacceptor polypeptide may include an amino acid sequence that has up to85%, 90%, 95%, or 99% identity to at least one of SEQ ID NOs:1, 2, 3, 4,5, 6, 7, 8, 9, or 10 and is a substrate for a lipoic acid ligase ormutant thereof. In some embodiments, an acceptor polypeptide includesthe amino acid sequence of SEQ ID NO: 8 or 10. An acceptor polypeptidemay be N- or C-terminally fused to a target protein. One of ordinaryskill in the art will understand how the amino acid sequence can bevaried and how to vary the sequence such that it functions as anacceptor polypeptide for the methods and compositions of the invention.Acceptor peptides can be synthesized using standard peptide synthesistechniques. One of ordinary skill in the art will also recognize how toprepare an acceptor polypeptide such that is it attached (fused) to atarget protein using routine methods.

One of ordinary skill in the art will recognize how to identify acceptorpolypeptides and how to modify acceptor polypeptides of the invention toprepare additional acceptor polypeptides that are useful in methods andcompositions of the invention. Various assays can be used to test thesequence specificity of acceptor polypeptides and their suitability formammalian cell labeling applications. A non-limiting example of a methodfor identifying an acceptor polypeptide includes combining a candidateacceptor polypeptide with a labeled lipoic acid or analog thereof in thepresence of a lipoic acid ligase or mutant thereof and determining alevel of lipoic acid or lipoic acid analog incorporation, wherein lipoicacid or lipoic acid analog incorporation is indicative of a candidateacceptor polypeptide having specificity for a lipoic acid ligase ormutant thereof.

The acceptor peptide is used in the methods of the invention to tagtarget proteins that are to be labeled by lipoic acid ligase and mutantsthereof. The acceptor peptide and target protein may be fused to eachother either at the nucleic acid or amino acid level. Recombinant DNAtechnology for generating fusion nucleic acids that encode both thetarget protein and the acceptor peptide are known in the art.Additionally, the acceptor peptide may be fused to the target proteinpost-translationally. Such linkages may include cleavable linkers orbonds which can be cleaved once the desired labeling is achieved. Suchbonds may be cleaved by exposure to a particular pH, or energy of acertain wavelength, and the like. Cleavable linkers are known in theart. Examples include thiol-cleavable cross-linker3,3′-dithiobis(succinimidyl proprionate), amine-cleavable linkers, andsuccinyl-glycine spontaneously cleavable linkers.

The acceptor peptide can be fused to the target protein at any position.In some instances, it is preferred that the fusion not interfere withthe activity of the target protein, accordingly, the acceptor peptide isfused to the protein at positions that do not interfere with theactivity of the protein. Generally, the acceptor peptides can be C- orN-terminally fused to the target proteins. In still other instances, itis possible that the acceptor peptide is fused to the target protein atan internal position (e.g., a flexible internal loop). These proteinsare then susceptible to specific tagging by lipoic acid ligase and/ormutants thereof in vivo and in vitro. This specificity is possiblebecause neither lipoic acid ligase nor the acceptor peptide react withany other enzymes or peptides in a cell.

The invention is also directed in part to the identification and use ofanalogs of lipoic acid in assays and methods of the invention such asthose described herein. As described herein, LplA naturally catalyzesthe ATP-dependent ligation of the small-molecule lipoic acid to aspecific lysine sidechain within one of three natural acceptor proteins(E2p, E2o, and. H-protein). As depicted in FIG. 5, LplA has beenredirected to ligate analogs of lipoic acid, in order to label proteinswith useful biophysical probes. A number of alkyl azide and alkyne LplAsubstrates of varying lengths have been synthesized (for example seeFIG. 5B).

The alkyl azides were synthesized either by nucleophilic substitution ofthe corresponding bromoalkanoic acid with sodium azide, or bymetal-catalyzed diazo transfer onto the amine precursor. The alkyneswere synthesized from the bromoalkanoic acid precursor, by displacementwith lithium acetylide. An HPLC assay has been developed to test ifthese, and other, substrate analogs could be used by LplA (FIG. 5C) andfound that all the tested alkyl azides and alkynes were incorporated tosome degree by LplA, with azides in general incorporated at a higherrate than alkynes (FIG. 5B). Among the alkyl azides a length dependencewas identified, with the n=7 azide. (azide 7) displaying the bestkinetics. In some embodiments of the invention, the carbon chain lengthof an alkyl azide (FIG. 7A) may be 5, 6, 7, 8, 9, or 10. In someembodiments, the carbon chain length of an alkyne of the invention (FIG.7B) may be 4, 5, 6, 7, or 8. In some embodiments, the carbon chainlength of an halide (FIG. 7C) of the invention may be 8, 9, 10 or 11,and in some embodiments, X═Br or Cl. As an example of characterizationof ligation, kinetic characterization of the ligation of azide 7 by LplAyielded a K_(cat) 0.111±0.003 s⁻¹ (only 2.3-fold higher than themeasured K_(cat) for lipoic acid ligation) and K_(m) of 127±11 μM (75-or 30-fold higher than the two reported K_(m) values for lipoic acid).In some embodiments, the alkyl azide is a modified alkyl azide and thealkyne carboxylic acid is a modified alkyne carboxylic acid. Thefollowing structures are non-limiting examples of core structures oflipoic acid analogs that may be used in methods and compositions of theinvention. These structures may be modified to prepare lipoic acidanalogs that are also useful in methods and products of the invention.Useful modifications include, but are not limited to, changing thelinker between the carboxylic acid and the azide/alkyne to make it morehydrophilic, such as introduction of one or two oxi groups. In someembodiments of the invention, a lipoic acid analog may be an aryl azide,diazirine, or benzophenone photoaffinity probe or a fluorophoresubstrate. A lipoic acid analog may be a4-azido-2,3,5,6-tetrafluorobenzoic derivative (FIG. 7E), a7,7′-azo-octanoic acid (FIG. 7G), a benzophenone (FIG. 7F) or a6,8-difluoro-7-hydroxycoumarin fluorophore derivative (FIG. 7D). In someembodiments of the invention, the aryl azide photoaffinity probe orfluorophore substrate may be a modified aryl azide photoaffinity probeor fluorophore substrate. Modifications may include changing the lengthof the alkyl chain that links the carboxylic acid to the photoaffinityprobe and/or fluorophore. The linker may also be modified to change itshydrophobicity properties by, for example, introducing oxi groups in thealkyl chain. Additionally, protected versions of the coumarin probe maybe used in which the hydroxyl and carboxylic acid functionalities areprotected with an acetate or acetoxymethylesther respectively in orderto render the molecule permeable to the cell membrane.

Exemplary structures of lipoic acid analogs that may be used in methodsof the invention include, but are not limited to:

One of ordinary skill in the art will recognize how to modify lipoicacid analogs of the invention to prepare additional lipoic acid analogsthat are useful in methods and compositions of the invention. Variousassays can be used to test the sequence specificity of LplA, and thesuitability of various lipoic acid analogs and acceptor polypeptides formammalian cell labeling applications. A non-limiting example of a methodfor identifying a lipoic acid analog having specificity for a lipoicacid ligase or a mutant includes combining an acceptor polypeptide witha candidate lipoic acid analog molecule in the presence of a lipoic acidligase or mutant thereof and determining the presence of lipoic acidanalog incorporation, wherein lipoic acid analog incorporation isindicative of a candidate lipoic acid analog having specificity for alipoic acid ligase or mutant thereof. Additional exemplary assays andmethods of determining the presence of lipoic acid incorporation areprovided in the Examples section herein.

In some aspects of the invention, an azide group that has been attachedto the target can be selectively derivatized to any fluorescent probeconjugated to a cyclooctyne reaction partner. The azide group is thususeful as a “functional group handle.” Direct ligation of a fluorophoremay be used as a labeling procedure, but incorporation of a “functionalgroup handle” is more feasible due to the small size of the lipoatebinding pocket, and provides greater versatility for subsequentincorporation of probes of any structure. Many functional group handleshave been used in chemical biology, including ketones, organic azides,and alkynes (Prescher, J. A. & Bertozzi, C. R. 2005 Nat. Chem. Biol. 1,13-21). Organic azides are suitable for live cell applications, becausethe azide group is both abiotic and non-toxic in animals and can beselectively derivatized under physiological conditions (without anyadded metals or cofactors) with cyclooctynes, which are also unnatural(Agard, N. J., et. al., 2006 ACS Chem. Biol. 1, 644-648). Methods ofusing functional group handles such as azides and alkynes are well knownin the art and methods and procedures for the use of such functionalgroup handles in combination with a cyclooctyne reaction a partner areunderstood and can be practiced by those of ordinary skill in the artusing routine techniques.

The invention is directed in part to generating lipoic acid ligasemutants that recognize lipoic acid analogs and conjugate such analogs tothe acceptor peptide. Lipoic acid ligase mutants can be generated in anynumber of ways, including in vitro compartmentalization, geneticselections, yeast display, or FACS in mammalian cells, described ingreater detail herein, all of which are standard methods understood androutinely practiced by those of ordinary skill in the art.

Labeling methods of the invention rely on the activity of lipoic acidligase and mutants thereof that recognize and conjugate lipoic acidanalogs onto fusion proteins via the acceptor peptide. The inventionprovides lipoic acid ligase mutants that recognize lipoic acid analogs.As used herein, a lipoic acid ligase mutant is a variant of lipoic acidligase that is enzymatically active towards a lipoic acid analog (suchas those described herein). As used herein, “enzymatically active” meansthat the mutant is able to recognize and conjugate a lipoic acid analogto the acceptor peptide.

A lipoic acid ligase mutant of the invention can have various mutations,including addition, deletion or substitution of one or more amino acids.Preferably, the mutation will be present in the lipoic acid interactionand activation region, spanning amino acids 16-149. Generally, thesemutants will possess one or more amino acid substitutions relative tothe wild-type lipoic acid ligase amino acid sequence (SEQ ID NO:11). Inmost instances, the lipoic acid ligase mutants do not comprise an aminoacid substitution (or other form of mutation) of the lysine thatcorresponds to lysine 133 of the wild-type E. coli lipoic acid ligaseset forth as SEQ ID NO:11 (which is the putative catalytic residue).

Some mutants were developed based on an analysis of the lipoic acidbinding site of wild-type lipoic acid ligase. Residues that appearimportant in the interaction with lipoic acid and/or lipoic acid analogsof the invention include nucleic acids that correspond to: N16, L17,V19, E20, E21, W37, F35, N41, R70, S71A, S72, H79, C85, T87, R140, F147,and H149 or wild-type E. coli lipoic acid ligase set forth as SEQ IDNO:11. Residues of wild-type E. coli lipoic acid ligase (set forth asSEQ ID NO:11) that influence lipoic acid or analog affinity include N16,L17, V19, E20, E21, W37, F35, N41, R70, S71A, S72, H79, C85, T87, R140,F147, and H149 (and the corresponding amino acid residues in ligases ofthe invention). In some important embodiments of the invention, mutantscomprise amino acid substitutions at one or more of the positions thatcorrespond to: N16, L17, V19, E20, E21, W37, W37 +S71, W37 +E20, W37+F35, F35, N41, R70, S71, S72, H79, C85, T87, R140, F147, and H149 ofwild-type E. coli lipoic acid ligase set forth as SEQ ID NO:11. Specificexamples of lipoic acid ligase mutants are proteins having at least oneof the amino acid substitution that corresponds to: N16A, L17A, V19A,E20A, E21A, W37A, W37G, W37S, W37V, W37A +S71A, W37A +E20A, W37L, W37I,W37T, W37N, W37V +E20G, W37V +F35A, W37V +E20A, F35A, N41A, R70A, S71A,S72A, H79A, C85A, T87A, R140A, F147A, H149A, and H149V of wild-type E.coli lipoic acid ligase set forth as SEQ ID NO:11. The inventioncontemplates the use of lipoic acid ligase mutants having an amino acidsubstitution at one or more of the aforementioned positions. Ofparticular importance in some embodiments, are lipoic acid ligasemutants that harbor amino acid substitutions at positions thatcorrespond to E20, F35, W37, S71, H79, F147 and H149 of SEQ ID NO:11.Examples include but are not limited to substitutions that correspond toE20A, W37A, W37G, W37S, W37V, W37L, W37N, W37I, W37T, W37V +E20G, W37V+E20A and W37V +F35A of SEQ ID NO:11.

A lipoic acid ligase mutant may retain some level of activity for lipoicacid or an analog thereof. Its binding affinity for lipoic acid or ananalog thereof may be similar to that of wild-type lipoic acid ligase.Preferably, the mutant has higher binding affinity for a lipoic acidanalog than it does for lipoic acid. Consequently, lipoic acidconjugation to an acceptor peptide would be lower in the presence of alipoic acid analog. In still other embodiments, the lipoic acid ligasemutant has no binding affinity for lipoic acid.

In some embodiments of the invention, a lipoic acid ligase analog mayhave a nucleic acid sequence that has up to 85%, 90%, 95%, or 99%identity to the nucleic acid sequence of a wild-type lipoic acid ligaseand ligates lipoic acid and/or a lipoic acid analog to an acceptorpolypeptide. A lipoic acid ligase analog (mutant) may include an aminoacid sequence that has up 85%, 90%, 95%, or 99% identity to the aminoacid sequence of wild-type E. coli lipoic acid ligase (e.g., to SEQ IDNO:11) and will retain function as a lipoic acid ligase in methods ofthe invention. In some embodiments, a lipoic acid ligase used in methodsof the invention is the lipoic acid ligase having the sequence set forthas SEQ ID NO:11.

One of ordinary skill in the art will recognize how to identify suitablelipoic acid ligases and how to modify lipoic acid ligases of theinvention to prepare additional lipoic acid ligases that are useful inmethods and compositions of the invention. Various assays can be used totest the specificity and functionality of a lipoic acid ligase and itssuitability for mammalian cell labeling applications. A non-limitingexample of a method for identifying a lipoic acid ligase includescontacting a lipoic acid or lipoic acid analog with an acceptorpolypeptide in the presence of a candidate lipoic acid ligase molecule,and detecting a lipoic acid or lipoic acid analog that is bound to theacceptor polypeptide, wherein the presence of a lipoic acid or lipoicacid analog bound to an acceptor polypeptide indicates that thecandidate lipoic acid ligase molecule is a lipoic acid ligase that hasspecificity for the lipoic acid or lipoic acid analog.

Lipoic acid incorporation can be measured using ³H-lipoic acid andmeasuring incorporation of radioisotope in the peptide. Conjugation ofthe lipoic acid analog to an acceptor peptide can be assayed by variousmethods including, but not limited to, HPLC or mass-spec assays, asdescribed herein and as shown in the figures herein.

The skilled artisan will realize that conservative amino acidsubstitutions may be made in lipoic acid ligase mutants to providefunctionally equivalent variants, i.e., the variants retain thefunctional capabilities of the particular lipoic acid ligase mutant. Asused herein, a “conservative amino acid substitution” refers to an aminoacid substitution that does not alter the relative charge or sizecharacteristics of the protein in which the amino acid substitution ismade. Variants can be prepared according to methods for alteringpolypeptide sequence known to one of ordinary skill in the art such asare found in references which compile such methods, e.g. MolecularCloning: A Laboratory Manual, J. Sambrook, et al., eds., Second Edition,Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989, orCurrent Protocols in Molecular Biology, F. M. Ausubel, et al., eds.,John Wiley & Sons, Inc., New York. Conservative substitutions of aminoacids include substitutions made amongst amino acids within thefollowing groups: (a) M, I, L, V; (b) F, Y, W; (c) K, R, H; (d) A, G;(e) S, T; (f) Q, N; and (g) E, D.

Conservative amino-acid substitutions in the amino acid sequence oflipoic acid ligase mutants to produce functionally equivalent variantstypically are made by alteration of a nucleic acid encoding the mutant.Such substitutions can be made by a variety of methods known to one ofordinary skill in the art. For example, amino acid substitutions may bemade by PCR-directed mutation, site-directed mutagenesis according tothe method of Kunkel (Kunkel, PNAS 82: 488-492, 1985), or by chemicalsynthesis of a nucleic acid molecule encoding a lipoic acid ligasemutant.

Similarly, lipoic acid ligase mutants can be made using standardmolecular biology techniques known to those of ordinary skill in theart. For example, the mutants may be formed by transcription andtranslation from a nucleic acid sequence encoding the mutant. Suchnucleic acid sequences can be made based on the teaching of wild-typelipoic acid ligase sequence and the position and type of amino acidsubstitution.

The invention further provides methods for screening candidate moleculesfor activity as a lipoic acid ligase mutant. These screening methods canalso be combined with methods for generating candidates. Exemplarymethods include, but are not limited to, in vitro compartmentalization,life/death selections in bacteria, yeast display, or FACS in mammaliancells, each of which is known and routinely used by those of ordinaryskill in the art. In vitro compartmentalization (IVC) selection strategyprovides a platform to conduct multiple turnover selection for enzymes.In this completely in vitro system genes are compartmentalized byforming a water-in-oil emulsion. In this water-in-oil emulsioncompartment genotype-phenotype linkage is maintained through out theentire process from transcription/translation to substrate to productformation. The main advantage of IVC over other traditional methods ofselection is its ability to select out faster enzymes from slowerenzymes.

The following is an example of a genetic selection strategy that may beused to evolve lipoic acid ligase mutants. In the method, the selectionis based on an E. coli strain with knock out LplA and LipB gene. Thisallows the strain to grow only in presence of succinate plus acetate orby introducing a functional LplA mutant that recognizes exogenous lipoicacid as its substrate. For selection an LplA mutant library may betransformed to this strain and will allow it to grow in presence of asuitable molar ratio of lipoic acid and its analog. Mutants thatrecognize lipoic acid will grow but mutants that do not recognize lipoicacid will cease to grow. β-lactam-based antibiotic will selectively killthe dividing bacteria (carrying mutants that are not of interest). Theremaining static pool of bacteria (carrying LplA mutants that are ofinterest) are harvested and used for successive round of selections.

The labeling methods of the invention further rely on lipoic acidanalogs that are recognized and conjugated to acceptor peptides bylipoic acid ligase mutants. As used herein, a lipoic acid analog is amolecule that may be structurally similar to lipoic acid. Lipoic acidanalogs may share one particular structural feature in common withlipoic acid. A lipoic acid analog may be synthesized from lipoic acid,but is not so limited. Examples of lipoic acid analogs include, but arenot limited to, an alkyl azide, an alkyne carboxylic acid, an aryl azidephotoaffinity probe, a fluorophore (coumarin) substrate, a modifiedalkyl azide, a modified alkyne, a carboxylic acid, a4-azido-2,3,5,6-tetrafluorobenzoic derivative, a 7,7′-azo-octanoic acid,a benzophenone, or a 6,8-difluoro-7-hydroxycoumarin fluorophorederivative (see FIG. 7 for exemplary derivatives).

The lipoic acid ligase mutants must be capable of recognizing andconjugating lipoic acid analogs to acceptor peptides, in a mannersimilar to that in which wild-type lipoic acid ligase recognizes andconjugates lipoic acid to the acceptor peptide.

The lipoic acid analog binds to a lipoic acid ligase mutant and itpreferably binds with an affinity comparable to the binding affinity ofwild-type lipoic acid ligase to lipoic acid. However, lipoic acidanalogs that bind with lower affinities are still useful according tothe invention. In some embodiments, the lipoic acid analog is notrecognized by wild-type lipoic acid ligase derived from either E. colior from other cell types (e.g., the cell in which the labeling reactionis proceeding).

Some lipoic acid analogs are not themselves directly detectable, whileothers are. In the case of the former type, the lipoic acid analogundergoes reaction with another moiety (after conjugation to theacceptor peptide). The subsequent modification of this former type oflipoic acid analog is referred to as a bio-orthogonal ligation reactionand it is used to couple (i.e., label) these lipoic acid analogs todetectable labels such as fluorophores.

FIG. 1 illustrates the synthesis of various azide and alkyne lipoic acidanalogs. These synthesis pathways are exemplary and other synthesisprotocols can be used to generate lipoic acid analogs for use in theinvention.

Accordingly, lipoic acid analogs that are not themselves directlydetectable must be reacted with a detectable moiety. Each lipoic acidanalog in this category will undergo a specific reaction dependent uponits functional groups and that of its reaction partner. Some of thesereactions include selective derivatization with a fluorescent probeconjugated to a cyclooctyne reaction partner as described above hereinand in the Examples section. It will be understood that the reactionpartner may comprise any detectable moiety and is not solely limited tofluorophores. FIG. 4 illustrates the synthetic routes tocyclooctyne-probe conjugates. These synthesis pathways are exemplary andother synthesis protocols can be used to generate conjugates for use inthe invention.

In some embodiments, a lipoic acid analog (e.g., an azide) may bereacted with phosphines in a Staudinger reaction. Azides and arylphosphines generally have no cellular counterparts. As a result, thereaction is quite specific. Azide variants with improved stabilityagainst hydrolysis in water at pH 6-8 are also useful in the methods ofthe invention. The alkyne/azide [3+2] cycloaddition chemistry, based onClick chemistry (Wang et al. J. Am. Chem. Soc. 125:11164-11165, 2003),is also specific, in part because the two reactive partners do not havecellular counterparts (i.e., the two functional groups are non-naturallyoccurring). Nonlimiting examples of fluorophores that may be conjugatedto a cyclooctyne are Alexa Fluor 568 and Cy3.

As stated above, other lipoic acid analogs may be themselves directlydetectable, e.g., comprise a detectable label. Examples of such lipoicacid analogs include but are not limited to those conjugated tocoumarin, fluorescein, aryl azides, diazirines, benzophenones,resorufins, various xanthene-type fluorophores, chloroalkanes,metal-binding ligands, or derivatives thereof.

A lipoic acid analog can also be fluorogenic. As used herein, afluorogenic compound is one that is not detectable (e.g., fluorescent)by itself, but when conjugated to another moiety becomes fluorescent. Anexample of this is non-fluorescent coumarin phosphine which reacts withazides to produce fluorescent coumarin. Fluorogenic lipoic acid analogsare especially useful to keeping background to a minimum (e.g., cellularimaging applications).

As stated above, the lipoic acid analogs can be conjugated to detectablelabels, e.g., through conjugation using a cyclooctyne reaction partner.A “detectable label” as used herein is a molecule or compound that canbe detected by a variety of methods including fluorescence, electricalconductivity, radioactivity, size, and the like. The label may be of achemical (e.g., carbohydrate, lipid, etc.), peptide or nucleic acidnature although it is not so limited. The label may be directly orindirectly detectable. The label can be detected directly for example byits ability to emit and/or absorb light of a particular wavelength. Alabel can be detected indirectly by its ability to bind, recruit and, insome cases, cleave (or be cleaved by) another compound, thereby emittingor absorbing energy. An example of indirect detection is the use of anenzyme label that cleaves a substrate into visible products.

The type of label used will depend on a variety of factors, such as butnot limited to the nature of the protein ultimately being labeled. Thelabel should be sterically and chemically compatible with the lipoicacid analog, the acceptor peptide and the target protein. In mostinstances, the label should not interfere with the activity of thetarget protein.

Generally, the label can be selected from the group consisting of afluorescent molecule, a chemiluminescent molecule (e.g.,chemiluminescent substrates), a phosphorescent molecule, a radioisotope,an enzyme, an enzyme substrate, an affinity molecule, a ligand, anantigen, a hapten, an antibody, an antibody fragment, a chromogenicsubstrate, a contrast agent, an MRI contrast agent, a PET label, aphosphorescent label, and the like.

Specific examples of labels include radioactive isotopes such as ³²P or³H; haptens such as digoxigenin and dintrophenyl; affinity tags such asa FLAG tag, an HA tag, a histidine tag, a GST tag; enzyme tags such asalkaline phosphatase, horseradish peroxidase, beta-galactosidase, etc.Other labels include fluorophores such as fluorescein isothiocyanate(“FITC”), Texas Red®, tetramethylrhodamine isothiocyanate (“TRITC”),4,4-difluoro-4-bora-3a, and 4a-diaza-s-indacene (“BODIPY”), Cy-3, Cy-5,Cy-7, Cy-Chrome™, R-phycoerythrin (R-PE), PerCP, allophycocyanin (APC),PharRed™, Mauna Blue, Alexa™ 350 and other Alexa™ dyes, and CascadeBlue®.

The labels can also be antibodies or antibody fragments or theircorresponding antigen, epitope or hapten binding partners. Detection ofsuch bound antibodies and proteins or peptides is accomplished bytechniques well known to those skilled in the art. Antibody/antigencomplexes which form in response to hapten conjugates are easilydetected by linking a label to the hapten or to antibodies whichrecognize the hapten and then observing the site of the label.Alternatively, the antibodies can be visualized using secondaryantibodies or fragments thereof that are specific for the primaryantibody used. Polyclonal and monoclonal antibodies may be used.Antibody fragments include Fab, F(ab)₂, Fd and antibody fragments whichinclude a CDR3 region. The conjugates can also be labeled using dualspecificity antibodies.

The label can be a positron emission tomography (PET) label such as 99mtechnetium and 18FDG.

The label can also be an singlet oxygen radical generator including butnot limited to resorufin, malachite green, fluorescein, benzidine andits analogs including 2-aminobiphenyl, 4-aminobiphenyl,3,3′-diaminobenzidine, 3,3′-dichlorobenzidine, 3,3′-dimethoxybenzidine,and 3,3′-dimethylbenzidine. These molecules are useful in EM stainingand can also be used to induce localized toxicity.

The label can also be an analyte-binding group such as but not limitedto a metal chelator (e.g., a copper chelator). Examples of metalchelators include EDTA, EGTA, and molecules having pyridiniumsubstituents, imidazole substituents, and/or thiol substituents. Theselabels can be used to analyze local environment of the target protein(e.g., Ca²⁺ concentration).

The label can also be a heavy atom carrier. Such labels would beparticularly useful for X-ray crystallographic study of the targetprotein. Heavy atoms used in X-ray crystallography include but are notlimited to Au, Pt and Hg. An example of a heavy atom carrier is iodine.

The label may also be a photoactivatable cross-linker. A photoactivablecross linker is a cross linker that becomes reactive following exposureto radiation (e.g., a ultraviolet radiation, visible light, etc.).Examples include benzophenones, aziridines, a photoprobe analog ofgeranylgeranyl diphosphate (2-diazo-3,3,3-trifluoropropionyloxy-farnesyldiphosphate or DATFP-FPP) (Quellhorst et al. J Biol. Chem. 2001 Nov. 2;276(44):40727-33), a DNA analogue5-[N-(p-azidobenzoyl)-3-aminoallyl]-dUTP (N(3)RdUTP),sulfosuccinimidyl-2(7-azido-4-methylcoumarin-3-acetamido)-ethyl-1,3′-dithiopropionate(SAED) and1-[N-(2-hydroxy-5-azidobenzoyl)-2-aminoethyl]-4-(N-hydroxysuccinimidyl)-succinate.

The label may also be a photoswitch label. A photoswitch label is amolecule that undergoes a conformational change in response toradiation. For example, the molecule may change its conformation fromcis to trans and back again in response to radiation. The wavelengthrequired to induce the conformational switch will depend upon theparticular photoswitch label. Examples of photoswitch labels includeazobenzene, 3-nitro-2-naphthalenemethanol. Examples of photoswitches arealso described in van Delden et al. Chemistry. 2004 Jan. 5; 10(1):61-70;van Delden et al. Chemistry. 2003 Jun. 16; 9(12):2845-53; Zhang et al.Bioconjug Chem. 2003 July-August;14(4):824-9; Irie et al. Nature. 2002Dec. 19-26; 420(6917):759-60; as well as many others.

The label may also be a photolabile protecting group. Examples ofphotolabile protecting group include a nitrobenzyl group, a dimethoxynitrobenzyl group, nitroveratryloxycarbonyl (NVOC),2-(dimethylamino)-5-nitrophenyl (DANP), Bis(o-nitrophenyl)ethanediol,brominated hydroxyquinoline, and coumarin-4-ylmethyl derivative.Photolabile protecting groups are useful for photocaging reactivefunctional groups.

The label may comprise non-naturally occurring amino acids. Examples ofnon-naturally occurring amino acids include for glutamine (Glu) orglutamic acid residues: α-aminoadipate molecules; for tyrosine (Tyr)residues: phenylalanine (Phe), 4-carboxymethyl-Phe, pentafluorophenylalanine (PfPhe), 4-carboxymethyl-L-phenylalanine (cmPhe),4-carboxydifluoromethyl-L-phenylalanine (F₂ cmPhe),4-phosphonomethyl-phenylalanine (Pmp),(difluorophosphonomethyl)phenylalanine (F₂Pmp), O-malonyl-L-tyrosine(malTyr or OMT), and fluoro-O-malonyltyrosine (FOMT); for prolineresidues: 2-azetidinecarboxylic acid or pipecolic acid (which have6-membered, and 4-membered ring structures respectively);1-aminocyclohexylcarboxylic acid (Ac₆c);3-(2-hydroxynaphtalen-1-yl)-propyl; S-ethylisothiourea;2-NH₂-thiazoline; 2-NH₂-thiazole; asparagine residues substituted with3-indolyl-propyl at the C terminal carboxyl group. Modifications ofcysteines, histidines, lysines, arginines, tyrosines, glutamines,asparagines, prolines, and carboxyl groups are known in the art and aredescribed in U.S. Pat. No. 6,037,134. These types of labels can be usedto study enzyme structure and function.

The label may be an enzyme or an enzyme substrate. Examples of theseinclude (enzyme (substrate)): Alkaline Phosphatase (4-Methylumbelliferylphosphate Disodium salt; 3-Phenylumbelliferyl phosphate Hemipyridinesalt); Aminopeptidase (L-Alanine-4-methyl-7-coumarinylamidetrifluoroacetate; Z-L-arginine-4-methyl-7-coumarinylamide hydrochloride;Z-glycyl-L-proline-4-methyl-7-coumarinylamide); Aminopeptidase B(L-Leucine-4-methyl-7-coumarinylamide hydrochloride); Aminopeptidase M(L-Phenylalanine 4-methyl-7-coumarinylamide trifluoroacetate); Butyrateesterase (4-Methylumbelliferyl butyrate); Cellulase(2-Chloro-4-nitrophenyl-beta-D-cellobioside); Cholinesterase(7-Acetoxy-1-methylquinolinium iodide; Resorufin butyrate);alpha-Chymotrypsin, (Glutaryl-L-phenylalanine4-methyl-7-coumarinylamide);N—(N-Glutaryl-L-phenylalanyl)-2-aminoacridone;N—(N-Succinyl-L-phenylalanyl)-2-aminoacridone); Cytochrome P450 2B6(7-Ethoxycoumarin); Cytosolic Aldehyde Dehydrogenase (Esterase Activity)(Resorufin acetate); Dealkylase (O⁷-Pentylresorufin); Dopaminebeta-hydroxylase (Tyramine); Esterase (8-Acetoxypyrene-1,3,6-trisulfonicacid Trisodium salt; 3-(2 Benzoxazolyl)umbelliferyl acetate;8-Butyryloxypyrene-1,3,6-trisulfonicacid Trisodium salt;2′,7′-Dichlorofluorescin diacetate; Fluorescein dibutyrate; Fluoresceindilaurate; 4-Methylumbelliferyl acetate; 4-Methylumbelliferyl butyrate;8-Octanoyloxypyrene-1,3,6-trisulfonic acid Trisodium salt;8-Oleoyloxypyrene-1,3,6-trisulfonic acid Trisodium salt; Resorufinacetate); Factor X Activated (Xa) (4-Methylumbelliferyl4-guanidinobenzoate hydrochloride Monohydrate); Fucosidase,alpha-L-(4-Methylumbelliferyl-alpha-L-fucopyranoside); Galactosidase,alpha-(4-Methylumbelliferyl-alpha-D galactopyranoside); Galactosidase,beta-(6,8-Difluoro-4-methylumbelliferyl-beta-D-galactopyranoside;Fluorescein di(beta-D-galactopyranoside);4-Methylumbelliferyl-alpha-D-galactopyranoside;4-Methylumbelliferyl-beta-D-lactoside:Resorufin-beta-D-galactopyranoside;4-(Trifluoromethyl)umbelliferyl-beta-D-galactopyranoside;2-Chloro-4-nitrophenyl-beta-D-lactoside); Glucosaminidase,N-acetyl-beta-(4-Methylumbelliferyl-N-acetyl-beta-D-glucosaminideDihydrate); Glucosidase,alpha-(4-Methylumbelliferyl-alpha-D-glucopyranoside); Glucosidase,beta-(2-Chloro-4-nitrophenyl-beta-D-glucopyranoside;6,8-Difluoro-4-methylumbelliferyl-beta-D-glucopyranoside;4-Methylumbelliferyl-beta-D-glucopyranoside;Resorufin-beta-D-glucopyranoside;4-(Trifluoromethyl)umbelliferyl-beta-D-glucopyranoside); Glucuronidase,beta-(6,8-Difluoro-4-methylumbelliferyl-beta-D-glucuronide Lithium salt;4-Methylumbelliferyl-beta-D-glucuronide Trihydrate); Leucineaminopeptidase (L-Leucine-4-methyl-7-coumarinylamide hydrochloride);Lipase (Fluorescein dibutyrate; Fluorescein dilaurate;4-Methylumbelliferyl butyrate; 4-Methylumbelliferyl enanthate;4-Methylumbelliferyl oleate; 4-Methylumbelliferyl palmitate; Resorufinbutyrate); Lysozyme(4-Methylumbelliferyl-N,N′,N″-triacetyl-beta-chitotrioside);Mannosidase, alpha-(4-Methylumbelliferyl-alpha-D-mannopyranoside);Monoamine oxidase (Tyramine); Monooxygenase (7-Ethoxycoumarin);Neuraminidase (4-Methylumbelliferyl-N-acetyl-alpha-D-neuraminic acidSodium salt Dihydrate); Papain (Z-L-arginine-4-methyl-7-coumarinylamidehydrochloride); Peroxidase (Dihydrorhodamine 123); Phosphodiesterase(1-Naphthyl 4-phenylazophenyl phosphate; 2-Naphthyl 4-phenylazophenylphosphate); Prolyl endopeptidase(Z-glycyl-L-proline-4-methyl-7-coumarinylamide;Z-glycyl-L-proline-2-naphthylamide; Z-glycyl-L-proline-4-nitroanilide);Sulfatase (4-Methylumbelliferyl sulfate Potassium salt); Thrombin(4-Methylumbelliferyl 4-guanidinobenzoate hydrochloride Monohydrate);Trypsin (Z-L-arginine-4-methyl-7-coumarinylamide hydrochloride;4-Methylumbelliferyl 4-guanidinobenzoate hydrochloride Monohydrate);Tyramine dehydrogenase (Tyramine).

The labels can be attached to the lipoic acid analogs either before orafter the analog has been conjugated to the acceptor peptide, presumingthat the label does not interfere with the activity of lipoic acidligase. Labels can be attached to the lipoic acid analogs by anymechanism known in the art. Some of these mechanisms are alreadydescribed above for particular analogs. Other examples of functionalgroups which are reactive with various labels include, but are notlimited to, (functional group: reactive group of light emissivecompound) activated ester:amines or anilines; acyl azide:amines oranilines; acyl halide:amines, anilines, alcohols or phenols; acylnitrile:alcohols or phenols; aldehyde:amines or anilines; alkylhalide:amines, anilines, alcohols, phenols or thiols; alkylsulfonate:thiols, alcohols or phenols; anhydride:alcohols, phenols,amines or anilines; aryl halide:thiols; aziridine:thiols or thioethers;carboxylic acid:amines, anilines, alcohols or alkyl halides;diazoalkane:carboxylic acids; epoxide:thiols; haloacetamide:thiols;halotriazine:amines, anilines or phenols; hydrazine:aldehydes orketones; hydroxyamine:aldehydes or ketones; imido ester:amines oranilines; isocyanate:amines or anilines; and isothiocyanate:amines oranilines.

The labels are detected using a detection system. The nature of suchdetection systems will depend upon the nature of the detectable label.The detection system can be selected from any number of detectionsystems known in the art. These include a fluorescent detection system,a photographic film detection system, a chemiluminescent detectionsystem, an enzyme detection system, an atomic force microscopy (AFM)detection system, a scanning tunneling microscopy (STM) detectionsystem, an optical detection system, a nuclear magnetic resonance (NMR)detection system, a near field detection system, and a total internalreflection (TIR) detection system.

The invention provides in some instances lipoic acid ligase or mutantthereof and/or lipoic acid analogs in an isolated form. As used herein,an isolated lipoic acid ligase or mutant thereof is a lipoic acid ligaseor mutant thereof that is separated from its native environment insufficiently pure form so that it can be manipulated or used for any oneof the purposes of the invention. Thus, isolated means sufficiently pureto be used (i) to raise and/or isolate antibodies, (ii) as a reagent inan assay, or (iii) for sequencing, etc.

Isolated lipoic acid analogs similarly are analogs that have beensubstantially separated from either their native environment (if itexists in nature) or their synthesis environment. Accordingly, thelipoic acid analogs are substantially separated from any or all reagentspresent in their synthesis reaction that would be toxic or otherwisedetrimental to the target protein, the acceptor peptide, the lipoic acidligase mutant, or the labeling reaction. Isolated lipoic acid analogs,for example, include compositions that comprise less than 25%contamination, less than 20% contamination, less than 15% contamination,less than 10% contamination, less than 5% contamination, or less than 1%contamination (w/w).

The invention further provides nucleic acids coding for lipoic acidligase mutants. These nucleic acids therefore encode a lipoic acidligase mutant having an amino acid substitution at one or more of thefollowing residues of wild-type LplA (such as that set forth as SEQ IDNO:11) 16, 17, 19, 20, 21, 37, 37 +71, 37 +20, 37 +35, 35, 41, 70, 71,72, 79, 85, 87, 140, 147, and 149. Specific examples of amino acidsubstitutions may be one or more amino acids that correspond to: N16A,L17A, V19A, E20A, E21A, W37A, W37G, W37S, W37V, W37A +S71A, W37A +E20A,W37L, W37I, W37T, W37N, W37V +E20G, W37V +F35A, W37V +E20A, F35A, N41A,R70A, S71A, S72A, H79A, C85A, T87A, R140A, F147A, H149A, and H149V ofwild-type E. coli lipoic acid ligase.

The nucleotide sequence of wild-type lipoic acid ligase is provided asSEQ ID NO: 12. One of ordinary skill in the art will be able todetermine the codons corresponding to each of the amino acid residuesrecited herein.

The invention also embraces degenerate nucleic acids that differ fromthe mutant nucleic acid sequences provided herein in codon sequence dueto degeneracy of the genetic code. For example, serine residues areencoded by the codons TCA, AGT, TCC, TCG, TCT and AGC. Each of the sixcodons is equivalent for the purposes of encoding a serine residue.Thus, it will be apparent to one of ordinary skill in the art that anyof the serine-encoding nucleotide triplets may be employed to direct theprotein synthesis apparatus, in vitro or in vivo, to incorporate aserine residue into an elongating mutant. Similarly, nucleotide sequencetriplets which encode other amino acid residues include, but are notlimited to: CCA, CCC, CCG and CCT (proline codons); CGA, CGC, CGG, CGT,AGA and AGG (arginine codons); ACA, ACC, ACG and ACT (threonine codons);AAC and AAT (asparagine codons); and ATA, ATC and ATT (isoleucinecodons). Other amino acid residues may be encoded similarly by multiplenucleotide sequences.

The invention also involves expression vectors coding for lipoic acidligase mutants and host cells containing those expression vectors.Virtually any cells, prokaryotic or eukaryotic, which can be transformedwith heterologous DNA or RNA and which can be grown or maintained inculture, may be used in the practice of the invention. Examples includebacterial cells such as E. coli, mammalian cells such as mouse, hamster,pig, goat, primate, etc., and other eukaryotic cells such as Xenopuscells, Drosophila cells, Zebrafish cells, C. elegans cells, and thelike. They may be of a wide variety of tissue types, including mastcells, fibroblasts, oocytes and lymphocytes, and they may be primarycells or cell lines. Specific examples include CHO cells and COS cells.Cell-free transcription systems also may be used in lieu of cells.

As used herein, a “vector” may be any of a number of nucleic acids intowhich a desired sequence may be inserted by restriction and ligation fortransport between different genetic environments or for expression in ahost cell. Vectors are typically composed of DNA although RNA vectorsare also available. Vectors include, but are not limited to, plasmids,phagemids and virus genomes. A cloning vector is one which is able toreplicate in a host cell, and which is further characterized by one ormore endonuclease restriction sites at which the vector may be cut in adeterminable fashion and into which a desired DNA sequence may beligated such that the new recombinant vector retains its ability toreplicate in the host cell. In the case of plasmids, replication of thedesired sequence may occur many times as the plasmid increases in copynumber within the host bacterium or just a single time per host beforethe host reproduces by mitosis. In the case of phage, replication mayoccur actively during a lytic phase or passively during a lysogenicphase.

An expression vector is one into which a desired DNA sequence may beinserted by restriction and ligation such that it is operably joined toregulatory sequences and may be expressed as an RNA transcript. Vectorsmay further contain one or more marker sequences (i.e., reportersequences) suitable for use in the identification of cells which have orhave not been transformed or transfected with the vector. Markersinclude, for example, genes encoding proteins which increase or decreaseeither resistance or sensitivity to antibiotics or other compounds,genes which encode enzymes whose activities are detectable by standardassays known in the art (e.g., beta-galactosidase or alkalinephosphatase), and genes which visibly affect the phenotype oftransformed or transfected cells, hosts, colonies or plaques. Preferredvectors are those capable of autonomous replication and expression ofthe structural gene products present in the DNA segments to which theyare operably joined.

As used herein, a marker or coding sequence and regulatory sequences aresaid to be “operably” joined when they are covalently linked in such away as to place the expression or transcription of the coding sequenceunder the influence or control of the regulatory sequences. If it isdesired that the coding sequences be translated into a functionalprotein, two DNA sequences are said to be operably joined if inductionof a promoter in the 5′ regulatory sequences results in thetranscription of the coding sequence and if the nature of the linkagebetween the two DNA sequences does not (1) result in the introduction ofa frame-shift mutation, (2) interfere with the ability of the promoterregion to direct the transcription of the coding sequences, or (3)interfere with the ability of the corresponding RNA transcript to betranslated into a protein. Thus, a promoter region would be operablyjoined to a coding sequence if the promoter region were capable ofeffecting transcription of that DNA sequence such that the resultingtranscript might be translated into the desired protein or polypeptide.

The precise nature of the regulatory sequences needed for geneexpression may vary between species or cell types, but shall in generalinclude, as necessary, 5′ non-transcribed and 5′ non-translatedsequences involved with the initiation of transcription and translationrespectively, such as a TATA box, capping sequence, CCAAT sequence, andthe like. Especially, such 5′ non-transcribed regulatory sequences willinclude a promoter region which includes a promoter sequence fortranscriptional control of the operably joined coding sequence.Regulatory sequences may also include enhancer sequences or upstreamactivator sequences as desired. The vectors of the invention mayoptionally include 5′ leader or signal sequences. The choice and designof an appropriate vector is within the ability and discretion of one ofordinary skill in the art.

Expression vectors containing all the necessary elements for expressionare commercially available and known to those skilled in the art. See,e.g., Sambrook et al., Molecular Cloning: A Laboratory Manual, SecondEdition, Cold Spring Harbor Laboratory Press, 1989. Cells aregenetically engineered by the introduction into the cells ofheterologous nucleic acid, usually DNA, molecules, encoding a lipoicacid ligase mutant. The heterologous nucleic acid molecules are placedunder operable control of transcriptional elements to permit theexpression of the heterologous nucleic acid molecules in the host cell.

Preferred systems for mRNA expression in mammalian cells are those suchas pcDNA3.1 (available from Invitrogen, Carlsbad, Calif.) that contain aselectable marker such as a gene that confers G418 resistance (whichfacilitates the selection of stably transfected cell lines) and thehuman cytomegalovirus (CMV) enhancer-promoter sequences. Additionally,suitable for expression in primate or canine cell lines is the pCEP4vector (Invitrogen, Carlsbad, Calif.), which contains an Epstein Barrvirus (EBV) origin of replication, facilitating the maintenance ofplasmid as a multicopy extrachromosomal element. Another expressionvector is the pEF-BOS plasmid containing the promoter of polypeptideElongation Factor lcc, which stimulates efficiently transcription invitro. The plasmid is described by Mishizuma and Nagata (Nuc. Acids Res.18:5322, 1990), and its use in transfection experiments is disclosed by,for example, Demoulin (Mol. Cell. Biol. 16:4710-4716, 1996). Stillanother preferred expression vector is an adenovirus, described byStratford-Perricaudet, which is defective for E1 and E3 proteins (J.Clin. Invest. 90:626-630, 1992). The use of the adenovirus as anAdeno.P1A recombinant is disclosed by Warnier et al., in intradermalinjection in mice for immunization against PIA (Int. J. Cancer,67:303-310, 1996).

The invention also embraces so-called expression kits, which allow theartisan to prepare a desired expression vector or vectors. Suchexpression kits include at least separate portions of each of thepreviously discussed coding sequences. Other components may be added, asdesired, as long as the previously mentioned sequences, which arerequired, are included.

It will also be recognized that the invention embraces the use of theabove described, lipoic acid ligase mutant encoding nucleic acidcontaining expression vectors, to transfect host cells and cell lines,be these prokaryotic (e.g., E. coli), or eukaryotic (e.g., rodent cellssuch as CHO cells, primate cells such as COS cells, Drosophila cells,Zebrafish cells, Xenopus cells, C. elegans cells, yeast expressionsystems and recombinant baculovirus expression in insect cells).Especially useful are mammalian cells such as human, mouse, hamster,pig, goat, primate, etc., from a wide variety of tissue types includingprimary cells and established cell lines.

Various methods of the invention also require expression of fusionproteins in vivo. The fusion proteins are generally recombinantlyproduced proteins that comprise the lipoic acid ligase acceptorpeptides. Such fusions can be made from virtually any protein and thoseof ordinary skill in the art will be familiar with such methods. Furtherconjugation methodology is also provided in U.S. Pat. Nos. 5,932,433;5,874,239 and 5,723,584.

In some instances, it may be desirable to place the lipoic acid ligaseor mutant thereof and possibly the fusion protein under the control ofan inducible promoter. An inducible promoter is one that is active inthe presence (or absence) of a particular moiety. Accordingly, it is notconstitutively active. Examples of inducible promoters are known in theart and include the tetracycline responsive promoters and regulatorysequences such as tetracycline-inducible T7 promoter system, and hypoxiainducible systems (Hu et al. Mol Cell Biol. 2003 December;23(24):9361-74). Other mechanisms for controlling expression from aparticular locus include the use of synthetic short interfering RNAs(siRNAs).

As used herein with respect to nucleic acids, the term “isolated” means:(i) amplified in vitro by, for example, polymerase chain reaction (PCR);(ii) recombinantly produced by cloning; (iii) purified, as by cleavageand gel separation; or (iv) synthesized by, for example, chemicalsynthesis. An isolated nucleic acid is one which is readily manipulableby recombinant DNA techniques well known in the art. Thus, a nucleotidesequence contained in a vector in which 5′ and 3′ restriction sites areknown or for which polymerase chain reaction (PCR) primer sequences havebeen disclosed is considered isolated but a nucleic acid sequenceexisting in its native state in its natural host is not. An isolatednucleic acid may be substantially purified, but need not be. Forexample, a nucleic acid that is isolated within a cloning or expressionvector is not pure in that it may comprise only a tiny percentage of thematerial in the cell in which it resides. Such a nucleic acid isisolated, however, as the term is used herein because it is readilymanipulable by standard techniques known to those of ordinary skill inthe art.

As used herein, a subject shall mean an organism such as an insect, ayeast cell, a worm, a fish, or a human or animal including but notlimited to a dog, cat, horse, cow, pig, sheep, goat, chicken, rodente.g., rats and mice, primate, e.g., monkey. Subjects include vertebrateand invertebrate species. Subjects can be house pets (e.g., dogs, cats,fish, etc.), agricultural stock animals (e.g., cows, horses, pigs,chickens, etc.), laboratory animals (e.g., mice, rats, rabbits, etc.),zoo animals (e.g., lions, giraffes, etc.), but are not so limited.Methods of the invention may be used to introduce labels for MRI, PET,or multiphoton imaging, etc. into and for detection in live animals.Methods of the invention may be applied to living animals, for example,transgenic animals, thus subjects of the invention may be transgenicanimals.

The compositions, as described above, are administered in effectiveamounts for labeling of the target proteins. The effective amount willdepend upon the mode of administration, the location of the cells beingtargeted, the amount of target protein present and the level of labelingdesired.

The methods of the invention, generally speaking, may be practiced usingany mode of administration that is medically acceptable, meaning anymode that produces effective levels of the active compounds withoutcausing clinically unacceptable adverse effects. A variety ofadministration routes are available including but not limited to oral,rectal, topical, nasal, intradermal, or parenteral routes. The term“parenteral” includes subcutaneous, intravenous, intramuscular, orinfusion.

When peptides are used, in certain embodiments one desirable route ofadministration is by pulmonary aerosol. Techniques for preparing aerosoldelivery systems containing peptides are well known to those of skill inthe art. Generally, such systems should utilize components which willnot significantly impair the biological properties of the peptides orproteins (see, for example, Sciarra and Cutie, “Aerosols,” inRemington's Pharmaceutical Sciences, 18th edition, 1990, pp 1694-1712;incorporated by reference). Those of skill in the art can readilydetermine the various parameters and conditions for producing protein orpeptide aerosols without resort to undue experimentation.

Preparations for parenteral administration include sterile aqueous ornon-aqueous solutions, suspensions, and emulsions. Examples ofnon-aqueous solvents are propylene glycol, polyethylene glycol,vegetable oils such as olive oil, and injectable organic esters such asethyl oleate. Aqueous carriers include water, alcoholic/aqueoussolutions, emulsions or suspensions, including saline and bufferedmedia. Parenteral vehicles include sodium chloride solution, Ringer'sdextrose, dextrose and sodium chloride, lactated Ringer's or fixed oils.Intravenous vehicles include fluid and nutrient replenishers,electrolyte replenishers (such as those based on Ringer's dextrose), andthe like. Preservatives and other additives may also be present such as,for example, antimicrobials, anti-oxidants, chelating agents, and inertgases and the like. Lower doses will result from other forms ofadministration, such as intravenous administration. In the event that aresponse in a subject is insufficient at the initial doses applied,higher doses (or effectively higher doses by a different, more localizeddelivery route) may be employed to the extent that subject tolerancepermits. Multiple doses per day are contemplated to achieve appropriatesystemic levels of compounds.

The agents may be combined, optionally, with apharmaceutically-acceptable carrier. The term“pharmaceutically-acceptable carrier” as used herein means one or morecompatible solid or liquid filler, diluents or encapsulating substanceswhich are suitable for administration into a subject. The term “carrier”denotes an organic or inorganic ingredient, natural or synthetic, withwhich the active ingredient is combined to facilitate the application.The components of the pharmaceutical compositions also are capable ofbeing commingled with the molecules of the present invention, and witheach other, in a manner such that there is no interaction that wouldsubstantially impair the desired pharmaceutical efficacy.

The invention in other aspects includes pharmaceutical compositions.When administered, the pharmaceutical preparations of the invention areapplied in pharmaceutically-acceptable amounts and inpharmaceutically-acceptably compositions. Such preparations mayroutinely contain salt, buffering agents, preservatives, compatiblecarriers, and the like. When used in medicine, the salts should bepharmaceutically acceptable, but non-pharmaceutically acceptable saltsmay conveniently be used to prepare pharmaceutically-acceptable saltsthereof and are not excluded from the scope of the invention. Suchpharmacologically and pharmaceutically-acceptable salts include, but arenot limited to, those prepared from the following acids: hydrochloric,hydrobromic, sulfuric, nitric, phosphoric, maleic, acetic, salicylic,citric, formic, malonic, succinic, and the like. Also,pharmaceutically-acceptable salts can be prepared as alkaline metal oralkaline earth salts, such as sodium, potassium or calcium salts.

Various techniques may be employed for introducing nucleic acids of theinvention into cells, depending on whether the nucleic acids areintroduced in vitro or in vivo in a host. Such techniques includetransfection of nucleic acid-CaPO₄ precipitates, transfection of nucleicacids associated with DEAE, transfection with a retrovirus including thenucleic acid of interest, liposome mediated transfection, and the like.For certain uses, it is preferred to target the nucleic acid toparticular cells. In such instances, a vehicle used for delivering anucleic acid of the invention into a cell (e.g., a retrovirus, or othervirus; a liposome) can have a targeting molecule attached thereto. Forexample, a molecule such as an antibody specific for a surface membraneprotein on the target cell or a ligand for a receptor on the target cellcan be bound to or incorporated within the nucleic acid deliveryvehicle. For example, where liposomes are employed to deliver thenucleic acids of the invention, proteins which bind to a surfacemembrane protein associated with endocytosis may be incorporated intothe liposome formulation for targeting and/or to facilitate uptake. Suchproteins include capsid proteins or fragments thereof tropic for aparticular cell type, antibodies for proteins which undergointernalization in cycling, proteins that target intracellularlocalization and enhance intracellular half life, and the like.Polymeric delivery systems also have been used successfully to delivernucleic acids into cells, as is known by those skilled in the art. Suchsystems even permit oral delivery of nucleic acids.

Other delivery systems can include time-release, delayed release orsustained release delivery systems. Such systems can avoid repeatedadministrations of the labeling reagents. Many types of release deliverysystems are available and known to those of ordinary skill in the art.They include polymer base systems such as poly(lactide-glycolide),copolyoxalates, polycaprolactones, polyesteramides, polyorthoesters,polyhydroxybutyric acid, and polyanhydrides. Microcapsules of theforegoing polymers containing drugs are described in, for example, U.S.Pat. No. 5,075,109. Delivery systems also include non-polymer systemsthat are: lipids including sterols such as cholesterol, cholesterolesters and fatty acids or neutral fats such as mono- di- andtri-glycerides; hydrogel release systems; sylastic systems; peptidebased systems; wax coatings; compressed tablets using conventionalbinders and excipients; partially fused implants; and the like. Specificexamples include, but are not limited to: (a) erosional systems in whichthe anti-inflammatory agent is contained in a form within a matrix suchas those described in U.S. Pat. Nos. 4,452,775, 4,667,014, 4,748,034 and5,239,660 and (b) diffusional systems in which an active componentpermeates at a controlled rate from a polymer such as described in U.S.Pat. Nos. 3,832,253, and 3,854,480.

A preferred delivery system of the invention is a colloidal dispersionsystem. Colloidal dispersion systems include lipid-based systemsincluding oil-in-water emulsions, micelles, mixed micelles, andliposomes. A preferred colloidal system of the invention is a liposome.Liposomes are artificial membrane vessels which are useful as a deliveryvector in vivo or in vitro. It has been shown that large unilamellarvessels (LUV), which range in size from 0.2-4.0 μm can encapsulate largemacromolecules. RNA, DNA, and intact virions can be encapsulated withinthe aqueous interior and be delivered to cells in a biologically activeform (Fraley, et al., Trends Biochem. Sci., (1981) 6:77). In order for aliposome to be an efficient gene transfer vector, one or more of thefollowing characteristics should be present: (1) encapsulation of thegene of interest at high efficiency with retention of biologicalactivity; (2) preferential and substantial binding to a target cell incomparison to non-target cells; (3) delivery of the aqueous contents ofthe vesicle to the target cell cytoplasm at high efficiency; and (4)accurate and effective expression of genetic information.

Liposomes may be targeted to a particular tissue by coupling theliposome to a specific ligand such as a monoclonal antibody, sugar,glycolipid, or protein. Liposomes are commercially available from GibcoBRL, for example, as LIPOFECTIN™ and LIPOFECTACE™, which are formed ofcationic lipids such as N-[1-(2, 3 dioleyloxy)-propyl]-N,N,N-trimethylammonium chloride (DOTMA) and dimethyl dioctadecylammoniumbromide (DDAB). Methods for making liposomes are well known in the artand have been described in many publications. Liposomes also have beenreviewed by Gregoriadis, G. in Trends in Biotechnology, (1985)3:235-241.

In one important embodiment, the preferred vehicle is a biocompatiblemicroparticle or implant that is suitable for implantation into themammalian recipient. Exemplary bioerodible implants that are useful inaccordance with this method are described in PCT Internationalapplication no. PCT/US/03307 (Publication No. WO 95/24929, entitled“Polymeric Gene Delivery System”). PCT/US/03307 describes abiocompatible, preferably biodegradable polymeric matrix for containingan exogenous gene under the control of an appropriate promoter. Thepolymeric matrix is used to achieve sustained release of the exogenousgene in the patient. In accordance with the instant invention, thefugetactic agents described herein are encapsulated or dispersed withinthe biocompatible, preferably biodegradable polymeric matrix disclosedin PCT/US/03307.

The polymeric matrix preferably is in the form of a microparticle suchas a microsphere (wherein an agent is dispersed throughout a solidpolymeric matrix) or a microcapsule (wherein an agent is stored in thecore of a polymeric shell). Other forms of the polymeric matrix forcontaining an agent include films, coatings, gels, implants, and stents.The size and composition of the polymeric matrix device is selected toresult in favorable release kinetics in the tissue into which the matrixis introduced. The size of the polymeric matrix further is selectedaccording to the method of delivery which is to be used. Preferably whenan aerosol route is used the polymeric matrix and agent are encompassedin a surfactant vehicle. The polymeric matrix composition can beselected to have both favorable degradation rates and also to be formedof a material which is bioadhesive, to further increase theeffectiveness of transfer. The matrix composition also can be selectednot to degrade, but rather, to release by diffusion over an extendedperiod of time.

In another important embodiment the delivery system is a biocompatiblemicrosphere that is suitable for local, site-specific delivery. Suchmicrospheres are disclosed in Chickering et al., Biotech. And Bioeng.,(1996) 52:96-101 and Mathiowitz et al., Nature, (1997) 386:410-414.

Both non-biodegradable and biodegradable polymeric matrices can be usedto deliver the agents of the invention to the subject. Biodegradablematrices are preferred. Such polymers may be natural or syntheticpolymers. Synthetic polymers are preferred. The polymer is selectedbased on the period of time over which release is desired, generally inthe order of a few hours to a year or longer. Typically, release over aperiod ranging from between a few hours and three to twelve months ismost desirable. The polymer optionally is in the form of a hydrogel thatcan absorb up to about 90% of its weight in water and further,optionally is cross-linked with multivalent ions or other polymers.

In general, agents are delivered using a bioerodible implant by way ofdiffusion, or more preferably, by degradation of the polymeric matrix.Exemplary synthetic polymers which can be used to form the biodegradabledelivery system include: polyamides, polycarbonates, polyalkylenes,polyalkylene glycols, polyalkylene oxides, polyalkylene terepthalates,polyvinyl alcohols, polyvinyl ethers, polyvinyl esters, poly-vinylhalides, polyvinylpyrrolidone, polyglycolides, polysiloxanes,polyurethanes and co-polymers thereof, alkyl cellulose, hydroxyalkylcelluloses, cellulose ethers, cellulose esters, nitro celluloses,polymers of acrylic and methacrylic esters, methyl cellulose, ethylcellulose, hydroxypropyl cellulose, hydroxy-propyl methyl cellulose,hydroxybutyl methyl cellulose, cellulose acetate, cellulose propionate,cellulose acetate butyrate, cellulose acetate phthalate, carboxylethylcellulose, cellulose triacetate, cellulose sulphate sodium salt,poly(methyl methacrylate), poly(ethyl methacrylate),poly(butylmethacrylate), poly(isobutyl methacrylate),poly(hexylmethacrylate), poly(isodecyl methacrylate), poly(laurylmethacrylate), poly(phenyl methacrylate), poly(methyl acrylate),poly(isopropyl acrylate), poly(isobutyl acrylate), poly(octadecylacrylate), polyethylene, polypropylene, poly(ethylene glycol),poly(ethylene oxide), poly(ethylene terephthalate), poly(vinylalcohols), polyvinyl acetate, poly vinyl chloride, polystyrene,polyvinylpyrrolidone, and polymers of lactic acid and glycolic acid,polyanhydrides, poly(ortho)esters, poly(butiric acid), poly(valericacid), and poly(lactide-cocaprolactone), and natural polymers such asalginate and other polysaccharides including dextran and cellulose,collagen, chemical derivatives thereof (substitutions, additions ofchemical groups, for example, alkyl, alkylene, hydroxylations,oxidations, and other modifications routinely made by those skilled inthe art), albumin and other hydrophilic proteins, zein and otherprolamines and hydrophobic proteins, copolymers and mixtures thereof. Ingeneral, these materials degrade either by enzymatic hydrolysis orexposure to water in vivo, by surface or bulk erosion.

Examples of non-biodegradable polymers include ethylene vinyl acetate,poly(meth)acrylic acid, polyamides, copolymers and mixtures thereof.

Bioadhesive polymers of particular interest include bioerodiblehydrogels described by H. S. Sawhney, C. P. Pathak and J. A. Hubell inMacromolecules, (1993) 26:581-587, the teachings of which areincorporated herein, polyhyaluronic acids, casein, gelatin, glutin,polyanhydrides, polyacrylic acid, alginate, chitosan, poly(methylmethacrylates), poly(ethyl methacrylates), poly(butylmethacrylate),poly(isobutyl methacrylate), poly(hexylmethacrylate), poly(isodecylmethacrylate), poly(lauryl methacrylate), poly(phenyl methacrylate),poly(methyl acrylate), poly(isopropyl acrylate), poly(isobutylacrylate), and poly(octadecyl acrylate).

In addition, important embodiments of the invention include pump-basedhardware delivery systems, some of which are adapted for implantation.Such implantable pumps include controlled-release microchips. Apreferred controlled-release microchip is described in Santini, J T Jr.,et al., Nature, 1999, 397:335-338, the contents of which are expresslyincorporated herein by reference.

Use of a long-term sustained release implant may be particularlysuitable for treatment of chronic conditions. Long-term release, as usedherein, means that the implant is constructed and arranged to deliverytherapeutic levels of the active ingredient for at least 30 days, andpreferably 60 days. Long-term sustained release implants are well-knownto those of ordinary skill in the art and include some of the releasesystems described above.

The invention will be more fully understood by reference to thefollowing examples. These examples, however, are merely intended toillustrate the embodiments of the invention and are not to be construedto limit the scope of the invention.

EXAMPLES Example 1 Introduction

The following are general synthetic methods used in the experimentsdescribed herein, including those of Example 2.

General Synthetic Methods

Reagents were purchased from Sigma-Aldrich (St, Louis, Mo.), Alfa Aesar(Ward Hill, Mass.), TCI America (Portland, Oreg.), Invitrogen (Carlsbad,Calif.), or GE Healthcare and used without further purification.Analytical thin layer chromatography was performed using 0.25 mm silicagel 60 F₂₅₄ plates and visualized with ninhydrin or bromocresol. Flashcolumn chromatography was carried out using silica gel (ICN SiliTech32-63D). Mass spectra were recorded on an Applied Biosystems 200 QTRAPMass Spectrometer (foster City, Calif.) using electrospray ionization.HPLC was performed on a Varian Prostar Instrument (Palo Alto, Calif.)equipped with an autosampler and photo-diode-array detector. Foranalytical HPLC, a reverse-phase 250×4.6 mm Microsorb-MV 300 C18 columnwas used. For preparative HPLC, a reverse-phase 250×10 mm Microsorb-MV100 C18 column was used. Chromatograms were recorded at 210 nm unlessotherwise noted. ¹H NMR spectra were recorded on a Varian Mercury 300MHz instrument. Chemical shifts are reported in delta (δ) units, partsper million (ppm), and referenced to the residual solvent peak. Couplingconstants (J) are reported in hertz (Hz). The following abbreviationsfor multiplets are used: s, singlet; dt, doublet of triplets; t,triplet; m, multiplet. Probes were stored as DMF solutions (3-5 M) at−20° C.

Synthesis of Alkyl Azide Probes (FIG. 1)

n=6 azide (9). To an ice-cooled solution of sodium azide (1.78 g, 27.5mmol) in water (4.5 mL) was added dichloromethane (7.5 mL). The biphasicmixture was stirred vigorously and trifluoromethanesulfonic anhydride(0.93 mL, 5.5 mmol) was added slowly over 5 minutes. The reaction wasallowed to proceed for 2 hours at 4° C. The aqueous layer was thenseparated from the organic phase and extracted twice withdichloromethane (2×25 mL). The combined dichloromethane extracts werewashed with saturated sodium carbonate solution and concentrated to 13mL under reduced pressure. This crude triflyl azide was used withoutfurther purification. 7-aminoheptanoic acid (0.35 g, 2.4 mmol) wasdissolved in water (7.8 mL) and combined with potassium carbonate (0.52g, 3.8 mmol) and copper sulfate pentahydrate (6.6 mg, 26.4 μmol).Methanol (15.6 mL) was added to dissolve the mixture, followed by crudetriflyl azide. The reaction was allowed to proceed at 21° C. overnight.The mixture was concentrated under reduced pressure, treated withNaH₂PO₄ buffer (45 mL, 250 mM, pH 6.2) and extracted with ethyl acetate(4×50 mL) to remove byproduct sulfonamide and excess triflyl azide. ThepH of the aqueous solution was further reduced to 2.0 using concentratedHCl. The product was extracted with ethyl acetate (4×50 mL), dried overmagnesium sulfate, and finally evaporated to dryness under reducedpressure to afford the desired product as a pale yellow oil (42 mg, 0.26mmol, 10.7%). ¹H NMR (CDCl₃): δ 3.27 (t, 2H, J=6.9), 2.37 (t, 2H,J=7.5), 1.62 (m, 4H), 1.39 (m, 4H). ESI-MS calculated for [M−H]⁻:170.22; observed 170.16.

General procedure for synthesis of n=5, 7-10 azides (10). To a solutionof the corresponding bromo-alkanoic acid (10 mmol of 6, 8, 9, 10 or11-bromo-alkanoic acid) in DMF (20 mL) was added sodium azide (0.98 g,15 mmol). The mixture was allowed to stir at 21° C. overnight. Theprogress of the reaction was monitored by thin layer chromatography(0-40% ethyl acetate in hexanes). DMF was removed under reduced pressureand the resulting residue was re-suspended in HCl (25 mL, 1 N) andsubsequently extracted with ethyl acetate (4×25 mL). The combinedorganic layers were dried over magnesium sulfate, and evaporated todryness under reduced pressure to afford the desired azido-alkanoic acidas a pale yellow oil. Typical yields ranged from 35-65%.Characterization data for n=5 azide (6-azidohexanoic acid). ¹H NMR(CDCl₃): δ 3.28 (t, 2H, J=6.9), 2.37 (t, 2H, J=7.5), 1.64 (m, 4H), 1.35(m, 2H). ESI-MS calculated for [M−H]⁺: 156.17; observed 156.12.Characterization data for n=7 azide (8-azidooctanoic acid). ¹H NMR(CDCl₃): δ 3.26 (t, 2H, J=6.8), 2.36 (t, 2H, J=7.5), 1.62 (m, 4H), 1.35(m, 6H). ESI-MS calculated for [M−H]⁺: 184.22; observed 184.14.Characterization data for n=8 azide (9-azidononanoic acid). ¹H NMR(CDCl₃) δ 3.25 (t, 2H, J=6.9), 2.34, (t, 2H, J=7.5), 1.60 (m, 4H), 1.32(m, 8H). ESI-MS calculated for [M−H]⁻: 198.25; observed 198.18.Characterization data for n=9 azide (10-azidodecanoic acid). ¹H NMR(CDCl₃) δ 3.25 (t, 2H, J=7.1), 2.35, (t, 2H, J=7.5), 1.61 (m, 4H), 1.31(m, 10H). ESI-MS calculated for [M−H]⁻: 212.28; observed 212.16.Characterization data for n=10 azide (11-azidoundecanoic acid). ¹H NMR(CDCl₃) δ 3.25 (t, 2H, J=7.1), 2.35, (t, 2H, J=7.5), 1.62 (m, 4H), 1.29(m, 12H). ESI-MS calculated for [M−H]⁻: 226.31; observed 226.20.

Synthesis of Alkyne Probes (FIG. 1)

Synthesis of n=5 and n=7 alkynes (11). To a solution of 10 mmol 6- or8-bromo-alkanoic acid in DMSO (23 mL) was added lithium acetylideethylenediamine (0.92 g, 10 mmol) slowly over a period of 5 minutes. Thereaction mixture was stirred overnight at 21° C. Subsequently, water (25mL) was added and the product extracted with dichloromethane (3×25 mL).Dichloromethane was removed under reduced pressure to yield the desiredalkynoic acid as a brownish solid. Yields ranged from 5-7%.Characterization data for n=5 alkyne (7-octynoic acid). ¹H NMR (CDCl₃) δ2.36 (t, 2H, J=7.1), 2.19, (t, 2H, J=6.9), 1.94 (t, 1H, J=2.6),1.69-1.34 (m, 6H). ESI-MS calculated for [M−H]⁻: 139.18; observed139.08. Characterization data for n=7 alkyne (9-decynoic acid). ¹H NMR(CDCl₃) δ 2.35 (t, 2H, J=7.5), 2.18, (dt, 2H, J=6.9, J=2.7), 1.94 (t,1H, J=2.7), 1.63 (m, 2H), 1.51 (m, 2H), 1.35 (m, 6H) ESI-MS calculatedfor [M−H]⁻: 167.23; observed 167.22.

Synthesis of 5-(4-azido-2,3,5,6-tetrafluorobenzamido) pentanoic acid: Toa solution of 5-aminovaleric acid (12 mg, 100 μmol) in dry DMSO (250 μL)was added N-succinimidyl 4-azido-2,3,5,6-tetrafluorobenzoate (25 mg, 75μmol). The reaction was allowed to proceed overnight at 45° C. in thedark. The mixture was then acidified with 250 μL of 0.5 N hydrochloricacid and extracted with 400 μL of ethyl acetate three times. Thecombined extracts were concentrated in vacuo and purified by silicachromatography using 2:1 hexane-ethyl acetate to yield the desiredproduct as a white powder. Yielded: 3 mg, 9 μmol, 12%. TLC: R_(f)=0.3(1:1 hexane-ethyl acetate) ¹H-NMR (300 MHz, (CD₃)₂SO): δ 12.03 (s, 1H),8.89 (t, 1H, J=5.4), 3.23 (q, 2H, J=6.3), 2.22 (t, 2H, 0.1=7.2), 1.497(m, 4H). ESI-MS m/z: (neg) 333.07 [M−H]⁻. Calc. 334.23

Synthesis of 5-(3-amido-6,8-difluoro-7-hydroxycoumarin) pentanoic acid:To a solution of 5-aminovaleric acid (1.7 mg, 14.6 mmol) in anhydrousDMSO (200 μL) was added triethylamine (2.2 μL, 14.6 μmol) and stirredfor 5 mins. The N-hydroxy succinimidyl ester of3-carboxy-6,8-difluoro-7-hydroxycoumarin (2.5 mg, 7.3 μmol) was thenadded and the reaction was allowed to proceed for 4 hours at 21° C.Estimated yield 70%. ESI-MS: calculated for [M−H]⁻: 340.07; observed340.26.

Engineering a Peptide Substrate for LplA.

To rationally design a peptide substrate, we examined the NMR structureof E2p, which presents the lysine lipoylation site at the tip of a sharpβ-turn (1). Mutagenesis studies have shown that, while accuratepositioning of the target lysine within the β-turn is essential for LplArecognition, the residues flanking the lysine can be varied (2). Wedesigned three candidate peptides, peptides 4, 6, and 7 in FIG. 3;(lysine lipoylation site is underlined) by copying the 17-amino acidstretch encompassing the lysine lipoylation sites in each of the threenatural substrate proteins E2p, E2o, and H-protein. Two more peptides(shown in FIG. 3) were designed to resemble the natural proteinsubstrate (BCCP, peptide 1) and artificial peptide substrate (AP,peptide 2) of the mechanistically related enzyme E. coli biotin ligase(BirA). E. coli BCCP and E2p have similar overall folds, with similarpositioning of their acceptor lysines within β-turns (2). We also testedpeptides derived from a mutant of E. coli BCCP (peptide 3, FIG. 3) andfrom E2p of Bacillus stearothermophilus (peptide 5, FIG. 3), proteinsthat have previously been shown to be recognized by E. coli LplA (2,3).

These 7 peptides were cloned as N-terminal fusions to the protein HP1(4), expressed and purified, and tested using our HPLC assay(measurements of % conversion were performed in triplicate). The tablein FIG. 3 shows that three peptides were significantly lipoylated bywild-type LplA, with peptide 6 derived from E. coli E2o giving the bestconversion. To see if we could further improve peptide 6, we introducedseveral point mutations (data not shown). Only one of these, theVal(+1)Ala mutant (peptide 8), gave an increase in ligation rate. Wemeasured the k_(cat) for LplA-catalyzed ligation of azide 7 to peptide 8(0.048±0.001 s⁻¹, data not shown), and determined that it was only2.3-fold slower than the corresponding ligation k_(cat) for E2p (FIG.2B). In FIG. 3, conversion values are reported relative to peptide 8,whose percent conversion is normalized to 100%.

To test the transposability of peptide 8, we fused it to the C-terminalend of HP1 (to give peptide 9). The lipoylation rate dropped 5-fold, butwas then partially recovered by adding five extra C-terminal amino acidsto give peptide 10. This final 22-amino acid sequence was named the LplAacceptor peptide, or LAP, and used in all subsequent experiments.

Synthesis of OCT-Fluorophore Conjugates (FIG. 4)

Synthesis of OCT-PEG (3). To a solution of OCT acid 1⁵ (32 mg, 123 μmol)in dry dichloromethane (DCM, 750 μL) was added triethylamine (TEA, 34μL, 246 μmol). The mixture was stirred for 5 minutes at ambienttemperature. Pentafluorophenyl trifluoroacetate (PFP-TFA, 42 μL, 246μmol) was added slowly to the reaction mixture over 3 minutes and thereaction was allowed to proceed for 3 hours. The reaction mixture wasconcentrated in vacuo, then purified by silica chromatography (10% ethylacetate in hexane) to afford OCT-PFP 2 as a colorless solid (45.6 mg,107 μmol, 87.3%). The solid was immediately dissolved in dry DCM (1 mL),and combined with TEA (29 μL, 214 μmol).O,O′-Bis(3-aminopropyl)diethylene glycol (diamino-PEG, 71 μL, 535 μmol)was added, and the reaction mixture was allowed to stir overnight at 21°C. The crude mixture was purified on a silica flash column (5:3:1 ethylacetate:methanol:water with 4.3% triethylamine). The purified productwas concentrated under reduced pressure to yield OCT-PEG 3 (27.8 mg, 60μmol, 56%). ESI-MS calculated for [M+H]⁺: 463.29; observed 463.44.

Synthesis of OCT-PEG-Alexa Fluor 568 (4) and OCT-PEG-Cy3 (5). To asolution of OCT-PEG 3 (0.6 mg, 1.3 μmol) in anhydrous DMSO (200 μL) wasadded TEA (0.6 μL, 4 μmol). The N-hydroxy succinimidyl ester of eitherCy3 or Alexa Fluor 568 (1.3 μmol) was then added and the reaction wasallowed to stir for 4 hours at 21° C. The crude product was purified byHPLC on a reverse phase column. Chromatograms were recorded both at 210nm and 550 nm. The following conditions were used for elution: 30-80%acetonitrile in water over 30 minutes; flow rate 5.0 mL/minute. Solventwas removed in vacuo to afford the OCT-PEG-fluorophore conjugates.Estimated yield 50-65%. ESI-MS for OCT-PEG-Alexa Fluor 568 (4):calculated for [M−H]⁻: 1137.42; observed 1137.54. ESI-MS for OCT-PEG-Cy3(5): calculated for [M+H]⁺: 1075.48; observed 1075.08. In addition, thepresence of an intact OCT moiety was confirmed by reaction of 100 μMOCT-PEG-Alexa Fluor 568 (4) or OCT-PEG-Cy3 (5) with 1 mM azide 7 in DMSOat 30° C. for 6 hours and detection of the triazole cycloadduct productby MS. ESI-MS for OCT-PEG-Alexa Fluor 568-cycloadduct: calculated for[M−H]⁻: 1322.54; observed 1322.22. ESI-MS for OCT-PEG-Cy3-cycloadduct:calculated for [M−H]⁻: 1258.60; observed 1258.56.

Cloning of HP1-Peptide Fusions for Recombinant Expression in Bacteria

For peptides 1-7, the histone protein 1 (HP1) gene (4) was PCR-amplifiedusing a forward primer that introduced the desired peptide sequenceafter an NheI site (forward primer sequences below), and the reverseprimer HP1-EcoRI.R (5′ TTTT GAA TTC GGA TCC TTG CGG CTC GCC TCG TAC).The resulting PCR product was digested with NheI and EcoRI and ligatedin-frame to an NheI/EcoRI digested pET21b vector. The vector introduceda C-terminal His₆ tag.

Peptide Forward Primer Sequence (NheI sites are italicized) 1 5′AAAA GCT AGC GGC CTG AAC GAC ATC TTC GAA GCC GACAAA GCT GAA TGG CAC GAG GGC GGT GAG GAG GAG TACGCC GTG G (SEQ ID NO: 13) 2 5′AAAA GCT AGC GGC GAT ACC CTG TGC ATC GTT GAA GCCGAC AAA GCT GAA AAC CAG ATC GAA GGC GGT GAG GAGGAG TAC GCC GTG G (SEQ ID NO: 14) 3 5′AAAA GCT AGC GGC GAT ACC CTG TGC ATC GTT GAA GCCGAC AAA GCT TCT ATG GAA ATC CCG GGC GGT GAG GAGGAG TAC GCC GTG G (SEQ ID NO: 15) 4 5′AAAA GCT AGC GAA CAG TCG CTG ATC ACC GTA GAA GGCGAC AAA GCT TCT ATG GAA GTT CCG GGC GGT GAG GAGGAG TAC GCC GTG G (SEQ ID NO: 16) 5 5′AAAA GCT AGC GAC GAT GTA CTG TGC GAA GTA CAG AACGAC AAA GCT GTA GTT GAA ATC CCG GGC GGT GAG GAGGAG TAC GCC GTG G (SEQ ID NO: 17) 6 5′AAAA GCT AGC GAC GAA GTA CTG GTT GAA ATC GAA ACCGAC AAA GTA GTT CTG GAA GTA CCG GGC GGT GAG GAGGAG TAC GCC GTG G (SEQ ID NO: 18) 7 5′AAAA GCT AGC GGC GAT GAC TGC GCT GTT GCT GAA TCTGTA AAA GCT GCC TCG GAC ATC TAT GGC GGT GAG GAGGAG TAC GCC GTG G (SEQ ID NO: 19)Peptide 8 was obtained by mutagenesis on peptide 6, using the QuikChangeprimer 5′ GAA ATC GAA ACC GAC AAA GCA GTT CTG GAA GTA CCG GGC (SEQ IDNO:20) and its reverse complement. To clone peptide 9, the HP1 gene wasPCR-amplified using the primer NdeI-HP1.F (5′ AAAA CAT ATG GAG GAG GAGTAC GCC GTG G) (SEQ ID NO:21), which incorporates an NdeI site, and theprimer HP1-LAP-Stop-BamHI.R (5′ TTTT GGA TCC TCT TAC GGT ACT TCC AGA ACTGCT TTG TCG GTT TCG ATT TCA ACC AGT ACT TCG TCG CTA GCA TCC TTG CGG CTCGCC TCG TAC) (SEQ ID NO:22), which introduces the peptide sequence and aBamHI site. The resulting PCR product was digested with NdeI and BamHIand ligated in-frame to an NdeI/BamHI digested pET15b vector, whichintroduces an N-terminal His₆ tag. To clone peptide 10, the geneencoding peptide 9 was PCR-amplified using the same forward primerNdeI-HP1.F, and the reverse primer LAP-AAs-Stop-BamHI.R (5′ TTTT GGA TCCTCT TAC TCC TCC TCA CCG CCC GGT ACT TCC AGA ACT GCT TTG TC) (SEQ IDNO:23). The PCR product was digested and ligated as above.

Cloning of LAP-CFP for Cytoplasmic Mammalian Expression

The sequence for the LAP peptide was inserted in the NheI site of amodified form of the pcDNA3 vector (Invitrogen). The vector containedthe CFP gene between the BamHI and EcoRI sites (6). Primers 5′ AAAA ACTAGT CGG GCT GAC GAA GTA CTG GTT GAA ATC GAA ACC GAC AAA GCA GTT CTG GAAGTA CCG GCA TCA GCA GAC GGC GCT AGC AAAA (SEQ ID NO:24) and its reversecomplement were annealed together, digested with SpeI and NheI, andligated in-frame to NheI digested pcDNA3-CFP vector. To create theLAP(Ala)-CFP mutant, we performed QuikChange with the primer 5′G GTT GAAATC GAA ACC GAC GCC GCA GTT CTG GAA GTA CCG G (SEQ ID NO:25) and itsreverse complement.

Site Directed Mutagenesis of LplA

The pYFJ16 plasmid, a gift from John Cronan, encodes an N-terminallyHis₆-tagged LplA within the pQE-2 vector (Qiagen). To create thecatalytically inactive mutant, Lys133 was mutated to alanine byQuikChange using 5′ CGA AGG CGA CCG CGC AGT CTC AGG CTC GG (SEQ IDNO:26) and its reverse complement.

Cloning of LAP-CFP-TM for Mammalian Surface Expression

The LAP peptide was inserted between the BglII and AscI sites of theAP-CFP-TM plasmid⁶. Primers 5′ AAAAA AGA TCT GGC GGC GAC GAA GTA CTG GTTGAA ATC GAA ACC GAC AAA GCA GTT CTG GAA GTA CCG GGC GGT GAG GAG GAG GGCGCG CCA AAAA (SEQ ID NO:27) and its reverse complement were annealedtogether, digested with BglII and AscI, and ligated in-frame toBglII/AscI digested AP-CFP-TM. To create the LAP(Ala) mutant, weperformed QuikChange using the primer 5′ G GTT GAA ATC GAA ACC GAC GCCGCA GTT CTG GAA GTA CCG G (SEQ ID NO:28) and its reverse complement.

Cloning of LAP-LDLR and LAP-GFP-LDLR

To clone LAP-GFP-LDLR, the LAP sequence was inserted in the leadersequence of LDLR, between Ser27 and Thr28, using the inverse PCR methoddescribed by Gama and Breitwieser (12) with the primers 5′ CG GGC GGTGAG GAG GAG ACT GTG AGC AAG GGC GAG GAG (SEQ ID NO:29) and 5′ GTA CCTCCA GAA CTG CTT TGT CGG TTT CGA TTT CAA CCA GTA CTT CGT CAC TTC TGT CGCCAA CTG CAG (SEQ ID NO:30). The PCR template was the pEGFP-LDLR plasmid,a gift from Tom Kirchhausen. To clone LAP-LDLR, the GFP gene was deletedby QuikChange using primers 5′ GGA GGT ACC GGC ATC AGC AGA CGG CGG GGGAGA ATT CGA CAG ATG TG (SEQ ID NO:31) and its reverse complement.

Bacterial Expression and Purification of E. coli LplA

LplA was expressed from the plasmid pYFJ16, a gift from John Cronan,which encodes an N-terminally His₆-tagged LplA within the pQE-2 vector(Qiagen). pYFJ16 was transformed into E. coli BL21(DE3) cells, whichwere amplified in LB media supplemented with 100 μg/mL ampicillin at 37°C. until OD₆₀₀ 0.9. Enzyme expression was induced with 200 μg/mL IPTGfor 3 hours at 30° C. Thereafter, cells were harvested by centrifugation(6,000 rpm, 10 minutes, 4° C.) and the pellet was resuspended in lysisbuffer (50 mM Tris base, 300 mM NaCl, pH 7.8) containing 2.5 mMphenylmethylsulfonyl fluoride (PMSF) and protease inhibitor cocktail(Calbiochem). Cells were lysed by ultrasonic treatment (six 15-secondbursts, with 1 minute of cooling to 4° C. between bursts). The extractwas cleared by centrifugation (17,700 g, 10 minutes, 4° C.) and theHis₆-tagged enzyme was purified using Ni-NTA agarose (Qiagen). Fractionswere analyzed by 12% SDS-PAGE followed by Coomassie staining. Fractionscontaining LplA were pooled and dialyzed against PBS pH 7.4. Enzymeconcentration was determined by measuring A₂₈₀ and using the reportedextinction coefficient (46,250 M⁻¹ cm⁻¹) (13).

Bacterial Expression and Purification of E2p

A single hybrid lipoyl domain derived from the second subunit of the E.coli pyruvate dehydrogenase was expressed from the E. coli K12 strain,TM245, which was a gift from John Cronan (14,15). Transformants ofstrain TM245 were grown at 37° C. in LB media supplemented with 100μg/mL ampicillin until OD₆₀₀ 0.2. Protein expression was induced with 10μg/mL IPTG for 17 hours at 25° C. Harvested bacteria were resuspended in20 mM sodium phosphate buffer pH 7.4 containing 2 mM EDTA, 2.5 mM PMSF,and protease inhibitor cocktail (Calbiochem). Cells were lysed byultrasonic treatment (eight 30-second bursts, with 30 seconds of coolingto 4° C. between bursts). The extract was cleared by centrifugation(17,700 g, 40 minutes, 4° C.), before lowering the pH to 3.8 with 1 MHCl. Insoluble material was removed by two rounds of centrifugation(17,700 g, 20 minutes, 4° C. and 17,700 g, 10 minutes, 4° C.), beforeincreasing the pH to 7.0 with 1 M NaOH. The supernatant was dialyzedagainst 10 mM ammonium acetate pH 5.0, then subjected to fast flowanion-exchange chromatography on a 1 mL Q-Sepharose column with a 10-600mM ammonium acetate pH 5.0 gradient generated over 20 column volumes.Eluted fractions were analyzed by 19% SDS-PAGE followed by Coomassiestaining. Fractions containing E2p were pooled and dialyzed against PBSpH 7.4. Protein concentration was measured using the BCA (bicinchoninicacid) assay with BSA as the standard.

Bacterial Expression and Purification of HP1-Peptide Fusions

E. coli BL21(DE3) cells transformed with one of the HP1 expressionplasmids were amplified in LB media supplemented with 100 μg/mLampicillin at 37° C. until OD₆₀₀ 0.9. Protein expression was inducedwith 100 μg/mL IPTG for 4 hours at 30° C. Cells were harvested bycentrifugation and purified as described above for LplA. Purificationfractions were analyzed by 16% SDS-PAGE followed by Coomassie staining.Fractions containing HP1-peptide protein were pooled and dialyzedagainst PBS pH 7.4. Protein concentration was measured using the BCAassay with BSA as the reference standard.

Measurement of LplA-Probe Ligation Kinetics (FIG. 2B)

To measure the k_(cat) for LplA ligation of lipoic acid to E2p, 50 nMLplA was combined with 200 μM E2p, 750 μM lipoic acid, 1 mM ATP, and 2mM magnesium acetate in 25 mM sodium phosphate pH 7.0. The reaction wasinitiated with addition of 1 mM ATP to the pre-warmed (30° C.) mixture.The reaction was incubated at 30° C., and 90 μL aliquots were removedevery 5 minutes and quenched with 50 mM EDTA (final concentration).Samples were analyzed by C18 reverse-phase HPLC as described in the“Methods” section. Measurements were performed in triplicate. Acalibration curve was obtained that correlated the ratio of integratedHPLC peak areas (E2p:E2p-probe) to the actual protein ratio, in order tocompensate for differences in the extinction coefficients of E2p andlipoylated E2p. The amount of product obtained at each time point wasplotted against time, to obtain the V_(max) of the reaction. From this,the k_(cat) value was obtained using the equation V_(max)=k_(cat)[E]_(total).

To measure the K_(m) and k_(cat) for ligation of azide 7, 200 nM LplAwas combined with 200 μM E2p, 1 mM ATP, 2 mM magnesium acetate in 25 mMsodium phosphate pH 7.0, and various concentrations of azide 7 (25, 50,66, 100, 240, 360 and 750 μM). 90 μL aliquots were removed from the 30°C. reactions at 5 minutes intervals, up to 30 minutes, and quenched with50 mM EDTA (final concentration). Samples were analyzed by HPLC asabove. The amount of product obtained at each time point was plottedagainst time, to obtain the initial velocity for each concentration ofazide 7. The collection of initial velocities (V₀) was then plottedagainst azide 7 concentration (25-750 μM), and fit to theMichaelis-Menten equation (V₀=V_(max) [azide 7]/(K_(m)+[azide 7])),using Origin 6.1 software, to obtain the K_(m) for azide 7. From theV_(max) value, k_(cat) was calculated using the equation V_(max)=k_(cat)[E]_(total).

Mass-Spectrometric Analysis of E2p-Azide 7 Conjugate (FIG. 2A)

2 μM LplA was combined with 200 μM E2p, 350 μM azide 7, 1 mM ATP, and 2mM magnesium acetate in 25 mM sodium phosphate pH 7.0. The ligationreaction was allowed to proceed to completion by incubating at 30° C.for 2 hours. The reaction mixture was desalted by extensive dialysisagainst water (2×4 h, followed by overnight dialysis). Thereafter, thesample was diluted to a final concentration of 25 μM E2p-azide 7 in 50%methanol with 2% acetic acid. Mass spectra were recorded under thepositive enhanced multi-charge mode of an ESI-MS.

HPLC Analysis of LplA Modification of HP1-Peptide Fusions (FIG. 3)

To compare the conversion rates for different peptide substrates,reactions were assembled as follows: 1.5 μM LplA, 150 μM HP1-peptidefusion, 750 μM azide 7, 1 mM ATP, and 2 mM magnesium acetate in 25 mMsodium phosphate pH 7.0. Reactions were incubated at 30° C. for 2 hours,then quenched with 50 mM EDTA (final concentration), and subsequentlyanalyzed by C18 reverse-phase HPLC using a gradient of 30-45%acetonitrile in water with 0.1% trifluoroacetic acid over 20 minuteswith a 1 mL/minute flow rate. Retention times for unmodified HP1-peptidefusions ranged from 8-12 minutes, and shifted to 16-21 minutes afterligation to azide 7. Measurements were performed in triplicate. Theextent of modification was calculated from the ratio ofHP1-peptide-azide 7 peak area to the sum of (HP1-peptide+HP1-peptide-azide 7) peak areas. The conversion with HP1-peptide 8 wasnormalized to 100% and other conversions were reported relative to this.

Measurement of k_(cat) for LplA-Catalyzed Azide 7 Ligation to LAP

To measure the k_(cat) for LplA-catalyzed ligation of azide 7 to LAP,the reaction conditions were as follows: 2 μM LplA, 1.3 mM LAP-HP1(N-terminal fusion of peptide 10 to HP1), 750 μM azide 7, 1 mM ATP, and2 mM magnesium acetate in 25 mM sodium phosphate pH 7.0. The reactionwas initiated with addition of 1 mM ATP to the pre-warmed 30° C.mixture. The mixture was incubated at 30° C., and 10 μL aliquots wereremoved every 5 minutes and quenched with 50 mM EDTA (finalconcentration). Samples were analyzed by C18 reverse-phase HPLC asabove. The amount of product obtained at each time point was plottedagainst time, to obtain the V_(max) of the reaction. From this, thek_(cat) value was obtained using the equation V_(max)=k_(cat)[E]_(total).

Labeling of Cell Surface LAP-CFP-TM with OCT-Biotin

HEK 293T cells were transfected with the LAP-CFP-TM plasmid usingLipofectamine 2000. After 36-48 hours at 37° C., the cells were washedtwice with fresh growth media (DMEM supplemented with 10% FBS and 1%penicillin/streptomycin). Enzymatic ligation of azide 7 was performed incomplete growth media with 10 μM LplA, 350 μM azide 7, 1 mM ATP, and 5mM magnesium acetate for 15 minutes at 32° C. Cells were then rinsedthree times with growth media, and incubated for 15 minutes at 32° C.with 250 μM OCT-biotin. Thereafter, the cells were washed twice withgrowth media and incubated with streptavidin-Alexa Fluor 568 (30 μg/ml,prepared as previously described¹⁶) for 15 minutes at 21° C. The cellswere washed once with growth media at 21° C. and twice with ice-coldDPBS, pH 7.4, and imaged in the same buffer on a Zeiss Axiovert 200M asdescribed under “Methods”.

Labeling of Cell Surface AP-CFP-TM with BirA and Ketone

HEK 293T cells were transfected with AP-CFP-TM plasmid (6) usingLipofectamine 2000. After 36-48 hours at 37° C., the cells were washedtwice with DPBS pH 7.4 and labeling was performed as previously reported(6). Briefly, enzymatic ligation of ketone to AP-CFP-TM was performed inDPBS pH 7.4, with 0.2 μM BirA, 1 mM ketone 1 (6), 1 mM ATP, and 5 mMmagnesium acetate for 60 minutes at 32° C. Cells were then washed twicewith DPBS, pH 6.2 and incubated for 60 minutes at 16° C. (to reduceendocytosis) with 1 mM benzophenone-biotin-hydrazide (6) in DPBS pH 6.2.Thereafter, the cells were washed twice with ice-cold DPBS pH 7.4, andincubated with 30 μg/mL streptavidin-Alexa Fluor 568 in DPBS pH 7.4 and1% BSA for 15 minutes at 4° C. The cells were washed twice with DPBS pH7.4, and imaged in the same buffer as described above.

Labeling of Cell Surface Q2-CFP-TM with Transglutaminase

HEK 293T cells were transfected with Q2-CFP-TM plasmid (7) usingLipofectamine 2000. After 36-48 hours at 37° C., the cells were washedtwice with DMEM, and labeling was performed as previously reported (7).Briefly, enzymatic ligation of biotin cadaverine to Q2-CFP-TM wasperformed in DMEM, with 0.3 μM or 1 μM guinea pig liver transglutaminase(TGase, NZyme BioTec GmbH), 0.5 mM biotin cadaverine, and 12 mM CaCl₂for 30 minutes at 37° C. Cells were then washed twice with DPBS pH 7.4,and incubated for 15 minutes at 4° C. (to reduce endocytosis) with 30μg/mL streptavidin-Alexa Fluor 568 in DPBS pH 7.4, and 1% BSA. The cellswere washed three times with ice-cold DPBS pH 7.4, and imaged in thesame buffer as described above.

LplA labeling is superior to ketone/biotin ligase and transglutaminaselabeling in speed, sensitivity, and specificity.

We previously reported two other methods for site-specific labeling ofpeptide-fused cell surface proteins with small molecule probes. Inketone/biotin ligase labeling, a 15-amino acid “acceptor peptide” (AP)tag is site-specifically conjugated to a ketone analog of biotin by E.coli biotin ligase (BirA) (6). The ketone is then selectivelyderivatized with a hydrazide- or hydroxylamine-conjugated fluorophore.Transglutaminase labeling uses the guinea pig liver transglutaminase tocovalently ligate cadaverine-functionalized fluorophores to proteinsfused to a 7-residue glutamine-containing “Q2-tag” recognition sequencefor transglutaminase (7). We directly compared the speed, sensitivity,and specificity of LplA labeling to these methods in side-by-sidelabeling experiments.

LAP-CFP-TM-expressing HEK cells were labeled with LplA and azide 7 for15 minutes, followed by OCT-PEG-biotin for 15 minutes, followed bystreptavidin (SA)-Alexa Fluor 568 conjugate for 15 minutes.AP-CFP-TM-expressing HEK cells were labeled with BirA and ketone (6) for60 minutes, followed by biotin-benzophenone-hydrazide (6) for 60minutes, followed by streptavidin-Alexa Fluor 568 conjugate for 15minutes. This optimized 2 hour 15 minutes labeling protocol was used toachieve similar signal to background intensity ratios as seen inprevious LplA labeling experiments (total time 45 minutes). Due to thelong labeling time, as well as the reduced pH and temperature requiredfor the hydrazone formation step, many cells became unhealthy androunded. AP-CFP-TM-expressing HEK cells were labeled with BirA andketone (6) for 60 minutes, followed by Alexa Fluor 568 hydrazide for 20minutes. No fluorophore conjugation was observed, and increasing thetime of the second step or the concentration of Alexa Fluor 568hydrazide increased the non-specific background (data not shown). Thiscontrasts to the successful LplA-catalyzed fluorophore conjugationdescribed in Example 2. One explanation for the difference in labelingsensitivity of these two methods is the faster second-order rateconstant for the azide-alkyne [3+2] cycloaddition reaction compared toketone-hydrazide ligation (5,8)

Transglutaminase (TGase) labeling was performed on HEK cells expressingthe Q2-CFP-TM construct (7) under optimized conditions, using biotincadaverine with 0.3 μM or 1 μM transglutaminase (7) for 30 minutes,followed by detection with streptavidin-Alexa Fluor 568 for 15 minutes.Negative controls were performed with the alanine mutant of the Q2construct. Labeling was comparable to LplA ligation of OCT-biotin inspeed and sensitivity. However, the specificity of transglutaminaselabeling was highly variable and difficult to optimize. Specificlabeling is seen with 0.3 μM transglutaminase, when comparing theQ2-CFP-TM image to the Q2(Ala)-CFP-TM negative control image. However,increasing the enzyme concentration by only 3-fold resulted in the lossof labeling specificity (right panels). By contrast, LplA was completelyspecific for the LAP sequence at enzyme concentrations ranging from 1 μMto 20 μM.

Determination of LplA Labeling Sensitivity with LAP-CFP-TM

We used the wedge method to estimate the concentration of CFP in singlecells expressing the LAP-CFP-TM construct (6, 17). A wedge-shapedmicrochamber was constructed from three glass coverslips. The lengthalong the x direction was 6.5 mm and the height of the chamber(z-direction) increased linearly from 0 to 150 μm. The chamber wasfilled with a solution of 10 μM purified CFP in PBS pH 7.4. Thefluorescence of the wedge was imaged under conditions identical to thoseused for cellular imaging. We assumed an average cell thickness of 5 μmand therefore interpolated to the region of the wedge with thickness 5μm and used the fluorescence intensity measured there as a referencestandard for CFP concentration measured in single cells. Using the wedgefor comparison, we imaged two samples of HEK and HeLa cells expressingLAP-CFP-TM. Examination of the CFP channel images for cells that displayclear OCT-biotin labeling (signal to background ≧3:1) showed that theCFP concentrations ranged from 5 μM to >1 mM. We therefore concludedthat cells expressing as little as 5 μM LAP-CFP-TM could be labeled byour method. This represents an upper limit to the labeling sensitivityof our methodology.

Orthogonality Test for Two-Color Labeling with LplA and BirA

HEK 293T cells were either singly transfected with LAP-GFP-LDLR orco-transfected with a mixture of AP-EGFR (or AP-EphA3) and CFP-pcDNA3 ina 6:1 ratio using Lipofectamine 2000. 24 hours after transfection, thecells were re-plated together in a 1:1 ratio. After further incubationfor 24 hours at 37° C., the cells were washed twice with fresh growthmedia (DMEM with 10% FBS and 1% penicillin/streptomycin). Simultaneousenzymatic ligations of azide 7 and biotin were performed in completegrowth media with 5 μM BirA, 10 μM LplA, 50 μM biotin, 350 μM azide 7, 1mM ATP, and 5 mM magnesium acetate for 60 minutes at 32° C. Cells werethen rinsed three times with growth media, and incubated for 20 minutesat 21° C. with 200-400 μM OCT-Cy3. Biotin was detected withstreptavidin-QD655 (13 nM, Invitrogen) for 10 minutes at 4° C.Thereafter, the cells were washed once with ice-cold 1% BSA in DPBS, pH7.4 and twice more with ice-cold DPBS, pH 7.4. Labeled cells were imagedas described above. The GFP filter set was 495/20 excitation, 515dichroic, 530/30 emission; the QD655 filter set was 405/20, 585dichroic, and 655/20 emission. LAP-GFP-LDLR cells were specificallylabeled with Cy3, while AP-EGFR cells (indicated by CFP marker) werespecifically labeled with QD655.

REFERENCES FOR EXAMPLE 1 AND FIGS. 1-4

-   1. Green, J. D., Laue, E. D., Perham, R. N., Ali, S. T. &    Guest, J. R. Three-dimensional structure of a lipoyl domain from the    dihydrolipoyl acetyltransferase component of the pyruvate    dehydrogenase multienzyme complex of Escherichia coli. J. Mol. Biol.    248, 328-343 (1995).-   2. Reche, P. & Perham, R. N. Structure and selectivity in    post-translational modification: attaching the biotinyl-lysine and    lipoyl-lysine swinging arms in multifunctional enzymes. EMBO J. 18,    2673-2682 (1999).-   3. Dardel, F., Packman, L. C. & Perham, R. N. Expression in    Escherichia coli of a sub-gene encoding the lipoyl domain of the    pyruvate dehydrogenase complex of Bacillus stearothermophilus. FEBS    Lett. 264, 206-210 (1990).-   4. Saunders, W. S. et al. Molecular cloning of a human homologue of    Drosophila heterochromatin protein HP1 using anti-centromere    autoantibodies with anti-chromo specificity. J. Cell Sci. 104,    573-582 (1993).-   5. Agard, N. J., Baskin, J. M., Prescher, J. A., Lo, A. &    Bertozzi, C. R. A comparative study of bioorthogonal reactions with    azides. ACS Chem. Biol. 1, 644-648 (2006).-   6. Chen, I., Howarth, M., Lin, W. & Ting, A. Y. Site-specific    labeling of cell surface proteins with biophysical probes using    biotin ligase. Nat. Methods 2, 99-104 (2005).-   7. Lin, C. W. & Ting, A. Y. Transglutaminase-catalyzed site-specific    conjugation of small-molecule probes to proteins in vitro and on the    surface of living cells. J. Am. Chem. Soc. 128, 4542-4543 (2006).-   8. Nauman, D. A. & Bertozzi, C. R. Kinetic parameters for    small-molecule drug delivery by covalent cell surface targeting.    Biochim. Biophys. Acta 1568, 147-154 (2001).-   9. Alper, P. B., Hung, S. C. & Wong, C. H. Metal catalyzed diazo    transfer for the synthesis of azides from amines. Tetrahedron    Letters 37, 6029-6032 (1996).-   10. Scriven, E. F. V. & Turnbull, K. Azides: their preparation and    synthetic uses. Chemical Reviews 88, 297-368 (1998).-   11. Novis-Smith, W. & Beumel, J. Preparation of alkynes and    dialkynes by reaction of mono-halo and dihaloalkanes with lithium    acetylenide-ethylenediamine complex. Synthetic Communications    441-442 (1974).-   12. Gama, L. & Breitwieser, G. E. Generation of epitope-tagged    proteins by inverse polymerase chain reaction mutagenesis. Methods    Mol. Biol. 182, 77-83 (2002).-   13. Green, D. E., Morris, T. W., Green, J., Cronan, J. E., Jr. &    Guest, J. R. Purification and properties of the lipoate protein    ligase of Escherichia coli. Biochem. J. 309, 853-862 (1995).-   14. Morris, T. W., Reed, K. E. & Cronan, J. E., Jr. Lipoic acid    metabolism in Escherichia coli: the lplA and lipB genes define    redundant pathways for ligation of lipoyl groups to apoprotein. J.    Bacteriol. 177, 1-10 (1995).-   15. Ali, S. T. & Guest, J. R. Isolation and characterization of    lipoylated and unlipoylated domains of the E2p subunit of the    pyruvate dehydrogenase complex of Escherichia coli. Biochem. J. 271,    139-145 (1990).-   16. Howarth, M. et al. A monovalent streptavidin with a single    femtomolar biotin binding site. Nature Methods 3, 267-273 (2006).-   17. Adams, S. R. et al. New biarsenical ligands and tetracysteine    motifs for protein labeling in vitro and in vivo: synthesis and    biological applications. J. Am. Chem. Soc. 124, 6063-6076 (2002).

Example 2 Introduction

Live cell imaging is a powerful method for studying protein dynamics atthe cell surface, but conventional probes, such as antibodies andfluorescent ligands, are bulky, interfere with protein function (1,2),or dissociate after internalization (3,4). To overcome theselimitations, we developed a method to covalently tag any cell surfaceprotein with any chemical probe with remarkable specificity. Throughrational design, we re-directed a microbial lipoic acid ligase (LplA)(5) to specifically ligate an alkyl azide to an engineered LplA acceptorpeptide (LAP) tag. The alkyl azide is then selectively derivatized witha cyclooctyne (6) conjugated to any probe of interest. We demonstratethe utility of this method by first labeling LAP fusion proteinsexpressed on the surface of living mammalian cells with Cy3,AlexaFluor568, and biotin. Next, we combined LAP-tagging with ourpreviously reported tagging method (7,8) to simultaneously monitor thedynamics of two receptors, co-expressed in the same cell, with differentfluorophores. Using a wound-healing assay, we found that while the LDLreceptor maintains a uniform distribution on the cell surface, theephrin receptor EphA3 is polarized to the leading edge. This methodologyprovides general access to biochemical and imaging studies of cellsurface proteins, using small fluorophores introduced via a shortpeptide tag.

Methods

In vitro LplA Activity Assays.

LplA reactions contained 2 μM LplA, 200 μM E2p, 350 μM probe, 1 mM ATP,2 mM magnesium acetate, and 25 mM sodium phosphate pH 7.0. Reactionswere incubated at 30° C. for 30 minutes, and then quenched with EDTA(final concentration 50 mM). Conversion to product was determined byHPLC on a C₁₈ reverse-phase column with a 40-57% gradient ofacetonitrile in water with 0.1% trifluoroacetic acid over 20 minutes(flow rate 1.0 mL/minute). Unmodified E2p had a retention time of ˜12minutes while E2p-probe conjugates eluted at 15-18 minutes. Percentconversion to product was calculated from the ratio of the E2p-probepeak area to the sum of (E2p+E2p-probe) peak areas. All measurementswere performed in triplicate.

LplA Specificity Test on Mammalian Lysate.

Human embryonic kidney (HEK) 293T cells were transfected withLAP-CFP-pcDNA3 plasmid using Lipofectamine 2000 (1 μg DNA/well of a6-well plate). Lysates were generated 48 hours later by hypotonic lysisto minimize protease release, as follows. Cells were lifted from theplates, concentrated by centrifugation, and resuspended in 1 mM HEPES pH7.5, 5 mM magnesium chloride, 1 mM phenylmethylsulphonyl fluoride, andprotease inhibitor cocktail (Calbiochem). After incubation at 4° C. for10 minutes, the cells were lysed by vigorous vortexing for 2 minutes at21° C. Crude lysate was clarified by centrifugation, and stored at −80°C. Lysate was labeled by incubating at 30° C. for 10 hours with 25 mMsodium phosphate pH 7.0, 1 μM LplA, 250 μM azide 7, 1 mM ATP, and 4 mMmagnesium acetate. Thereafter, Staudinger ligation was performed byadding FLAG-phosphine (14) to a final concentration of 500 μM, andincubating at 30° C. for 16 hours. Each reaction sample was then dividedinto thirds. The first third was analyzed by 12% SDS-PAGE followed byWestern blotting with anti-FLAG(M2)-peroxidase antibody conjugate(Sigma, 1:1000 dilution). The second sample was analyzed by 12% SDS-PAGEfollowed by Coomassie staining. The last third was analyzed by 12%SDS-PAGE without boiling the samples, in order to prevent unfolding ofCFP, and in-gel fluorescence was visualized on a Storm 860 instrument(Amersham).

Live Cell Labeling with Fluorescent Probes.

HEK 293T cells were transfected with the LAP-CFP-TM expression plasmidusing Lipofectamine 2000. After 36-48 hours at 37° C., the cells werewashed twice with fresh growth media (Dulbecco's Modified Eagle's Mediumwith 10% fetal bovine serum and 1% penicillin/streptomycin). Enzymaticligation of azide 7 was performed in complete growth media with 10 μMLplA, 350 μM azide 7, 1 mM ATP, and 5 mM magnesium acetate for 60minutes at 32° C. Cells were rinsed three times with growth media, andincubated for 20 minutes at 21° C. with 200-400 μM OCT-Cy3 or 100-200 μMOCT-AlexaFluor568. Thereafter, the cells were washed once with growthmedia, twice with a 1% bovine serum albumin (BSA) solution in Dulbecco'sPhosphate-Buffered Saline (DPBS) pH 7.4, and twice more with DPBS alone.Labeled cells were imaged in the same buffer on a Zeiss Axiovert 200Minverted epifluorescence microscope using a 40× oil-immersion lens. CFP(420/20 excitation, 450 dichroic, 475/40 emission), Cy3 andAlexaFluor568 (560/20 excitation, 585 dichroic, 605/30) and differentialinterference contrast (DIC) images (630/10 emission) were collected andanalyzed using Slidebook software (Intelligent Imaging Innovations).Fluorescence images were normalized to the same intensity range.Acquisition times ranged from 10-250 milliseconds.

Two-Color Live Cell Labeling with LplA and Biotin Ligase.

HEK 239T cells were co-transfected with the LAP-LDLR and AP-EGFR (16)plasmids in a 1:2 ratio, or with the LAP-LDLR and AP-EphA3 (a gift fromM. Lackmann, Monash University) plasmids in a 2:1 ratio. 24 hours aftertransfection, the cells were wounded with a pipet tip and allowed toheal over 16-24 hours. For labeling, cells were washed twice withcomplete growth media, and then incubated with 5 μM BirA, 10 μM LplA, 50μM biotin, 350 μM azide 7, 1 mM ATP, and 5 mM magnesium acetate for 60minutes at 32° C. Cells were then rinsed three times with growth media,and incubated for 20 minutes at 21° C. with 200-400 μM OCT-Cy3. Biotinwas detected by staining with 50 μg/mL monovalentstreptavidin-AlexaFluor488 (8) for 10 minutes at 4° C. The cells werewashed once with ice-cold 1% BSA in DPBS pH 7.4, then twice withice-cold DPBS, before imaging in the same buffer using the configurationdescribed above. The AlexaFluor488 filter set was 495/20 excitation, 515dichroic, 530/30 emission.

Results and Discussion

Fluorescent labeling of cell surface proteins enables imaging of thetrafficking and function of receptors, channels, and transporters. Manyprotein labeling methods have been developed in recent years (9), butnone currently allows the covalent attachment of small fluorophores ofany structure onto cell surface proteins modified only by a smallpeptide tag, with short labeling times and with extremely highspecificity over a wide range of expression levels and labelingconditions. To address this shortcoming, we developed a new proteinlabeling method based on the E. coli enzyme lipoic acid ligase (LplA)(5). In E. coli, LplA catalyzes the ATP-dependent covalent ligation oflipoic acid to one of three proteins involved in oxidative metabolism[E2p, E2o, and H-protein (5)] (FIG. 5A, top). LplA naturally exhibitsextremely high sequence specificity, but previous work showing that theenzyme accepts octanoic acid, 6-thio-octanoic acid, and selenolipoicacid in place of lipoic acid (5) suggest that the small-molecule bindingsite has considerable plasticity. To harness LplA for fluorescentlabeling, we re-engineered the system in three stages. First, throughsynthesis and testing of ten different substrate analogs, we discoveredan alkyl azide substrate that can be efficiently used by LplA in placeof lipoic acid. Once ligated to the target protein, the azide functionalgroup can be selectively derivatized with any fluorescent probeconjugated to a cyclooctyne reaction partner (6) (FIG. 5A). Second, tocreate a minimally invasive tag to direct the ligation of the alkylazide, we engineered, through iterative cycles of rational design, a22-amino acid replacement for LplA's natural protein substrates, whichcan be fused to the N- or C-terminus of any protein of interest. Third,we tested the specificity of LplA in the mammalian cell context andfound no background labeling of endogenous proteins.

For the first stage of LplA engineering, we considered a range of smallmolecule structures to replace lipoic acid. Direct ligation of afluorophore would offer a simpler and shorter labeling procedure, butincorporation of a “functional group handle” is more feasible due to thesmall size of the lipoate binding pocket, and provides greaterversatility for subsequent incorporation of probes of any structure.Many functional group handles have been used in chemical biology,including ketones, organic azides, and alkynes (10). Organic azides arethe most suitable for live cell applications, because the azide group isboth abiotic and non-toxic in animals and can be selectively derivatizedunder physiological conditions (without any added metals or cofactors)with cyclooctynes, which are also unnatural (6). To test if LplA couldaccept an azide substrate in place of lipoic acid, we synthesized apanel of alkyl azide carboxylic acids of varying lengths (FIG. 1), andtested them for ligation onto a 9 kDa lipoyl domain derived from thefull-length E2p protein (11) (abbreviated “E2p”) using an HPLC assay. Asadditional probes of the lipoate binding pocket we also synthesized aseries of alkyne carboxylic acids (FIG. 1). FIG. 5B shows that allprobes were incorporated by LplA to some degree, but the efficiency ofligation exhibited a clear length-dependence, with azide 7 giving thefastest kinetics. FIG. 5Cc shows the HPLC trace associated with azide 7ligation to E2p, in addition to negative control reactions with LplA orATP omitted. We collected the product peak (starred) from the top traceand analyzed it by mass-spectrometry, which confirmed that one moleculeof azide 7 had been site-specifically conjugated to E2p (FIG. 2). Wealso measured the kinetics of azide 7 ligation to E2p (FIG. 2), andcompared the values to those of lipoic acid ligation. The k_(cat) valueswere only slightly different (0.111±0.003 s⁻¹ vs 0.253±0.003 s⁻¹) butthe K_(m) increased 75- or 30-fold for azide 7 (127±11 μM) compared tolipoic acid [1.7 μM (5) or 4.5 μM (12)]. As seen below, however, it isstraightforward and non-toxic to provide azide 7 at concentrationshigher than 127 μM for live cell labeling, thus maximizing the rate ofligation.

For the second stage of engineering, we wished to design a peptidesubstrate for LplA to replace the protein substrates. It was necessaryfor the peptide to be fully transposable (recognized when fused to theN- or C-terminal ends of any protein) and to be recognized by LplA withsimilar efficiency to the natural protein substrates. As described inFIG. 3, we accomplished this through multiple rounds of rational design.A major challenge was presented by the fact that E2p presents the lysinemodification site at the tip of a sharp hairpin turn (13), aconformation that is difficult to recapitulate in a peptide.Nevertheless, we designed an initial panel of peptides by analyzinglipoate acceptor proteins from different species, as well asstructurally-related biotin acceptor proteins. Peptides that were activein the initial screen were then improved through site-directedmutagenesis and tested for recognition at either terminus of a modelprotein. The final 22-amino acid sequence, called the LplA acceptorpeptide (LAP), had a k_(cat) of 0.048±0.001 s⁻¹, only 2.3-fold lowerthan the corresponding k_(cat) for full-length E2p.

Our third task was to assess the specificity of LplA in the mammaliancell context. To do this, we created a LAP fusion to cyan fluorescentprotein (CFP), and expressed it in human embryonic kidney (HEK) cells.HEK lysates were then labeled with LplA, azide 7, and ATP, and theligated azide was detected by western blot, after functionalization witha FLAG peptide via the Staudinger ligation (14). FIG. 6 shows that inthe presence of thousands of mammalian proteins in lysate, only LAP-CFPis labeled by LplA. The expression level of LAP-CFP is so low that itcannot be seen above endogenous proteins in the Coomassie-stained gel.Negative controls with LplA replaced by a catalytically inactive mutant,or LAP-CFP replaced by an alanine point mutant at the lysinemodification site, show that labeling depends on the presence of LplAand an intact LAP sequence. This experiment and the live cell labelingexperiments described below demonstrate that LplA is a remarkablyspecific enzyme at the cell surface, and possibly within the cytosol aswell.

To test our newly engineered small molecule and peptide substrates forLplA in the live cell context, we first created an artificial constructby fusing LAP to CFP, and then fusing this in turn to the extracellularside of the transmembrane (TM) domain of the PDGF receptor. We alsosynthesized conjugates of our previously reported mono-fluorinatedcyclooctyne (6) (OCT) to two bright, red-emitting, andmembrane-impermeant fluorophores, AlexaFluor568 and Cy3 (FIG. 4).

To perform labeling, LAP-CFP-TM was expressed in HEK cells, and 350 μMazide 7 was added in the presence of LplA for 1 hour, followed by one ofthe fluorophore-OCT conjugates for 20 minutes. A LAP-CFP fusion wastargeted to the cell surface using a transmembrane (TM) domain.Cell-surface LAP was first labeled with azide 7 by LplA, and theintroduced azide was then labeled with a cyclooctyne probe conjugated toCy3 or AlexaFluor568. Negative controls with azide 7 omitted from thelabeling reaction, or with the LAP-CFP-TM replaced by its alanine pointmutant.

The results from these studies showed that transfected cells (indicatedby CFP fluorescence) were labeled with AlexaFluor568 or Cy3, whileuntransfected cells were not labeled. Interestingly, labeling withAlexaFluor568 generated higher background than Cy3 labeling, due tofaster non-specific internalization of the probe. We performedadditional negative controls with omission of azide 7 or replacement ofLAP-CFP-TM by its alanine mutant, and observed no labeling in eithercase. Unlike sodium azide, organic azides such as the clinicallyapproved drug AZT (15) are not known to be toxic to cells, but wenevertheless examined the effect of 24-hour exposure to azide 7 onmitochondrial respiration, and found no effect at concentrations lessthan 750 μM.

We also compared the speed, sensitivity, and specificity of LplAlabeling to two other peptide-based labeling methods previouslydescribed by our lab. Biotin ligase (BirA)/ketone tagging makes use of aketone isostere of biotin, which can be functionalized with hydrazideconjugates to label proteins fused to a 15-amino acid “acceptor peptide”(AP) (16). Transglutaminase labeling attaches cadaverine-functionalizedfluorophores to a glutamine-containing peptide recognition sequence(17). For the comparison experiments, we used LplA to label LAP-CFP-TMwith azide 7, followed by OCT-biotin, and followed bystreptavidin-AlexaFluor568 to detect the biotin. A total labeling timeof only 20 minutes was required for all three steps, in order to achievea signal to background ratio>3:1. In contrast, BirA/ketone labeling ofan analogous AP-CFP-TM construct with a biotin-hydrazide compoundfollowed by streptavidin detection required 2 hours and 15 minutes toachieve a similar signal to background ratio. We also quantified thesensitivity of LplA labeling using the wedge method (18) and determinedthat cells expressing as little as 5 μM LAP-CFP-TM could be specificallylabeled with OCT-biotin, with a signal to background ratio>3:1. Similarexperiments, were performed and demonstrated that LplA is also superiorto transglutaminase, particularly in terms of labeling specificity undera wide range of conditions.

To illustrate the use of LplA labeling for imaging actual receptors, wecreated a LAP fusion to the low-density lipoprotein receptor (LDLR),which functions in the uptake of cholesterol in peripheral tissues ofthe body (19), and we established that LAP-LDLR could be labeled withOCT-Cy3 or OCT-biotin in HEK cells, even when expressed at levelsmatching endogenous LDLR (data not shown). We then wished to imageLAP-LDLR in the context of a biological assay. For many imaging studies,it is desirable to visualize two different receptors at once in the samecell, in order to compare their distribution or trafficking patterns. Todevelop this capability, we investigated the compatibility of LplAlabeling with BirA/streptavidin targeting. Unlike BirA/ketone labeling,BirA/streptavidin targeting (7,8) makes use of site-specific biotinligation onto AP-tagged proteins, followed by recognition withstreptavidin-fluorophore conjugates. While the use of streptavidinincreases the total size of the label, the femtomolar affinity of thebiotin-streptavidin interaction makes this labeling approach much fasterand much more sensitive than BirA/ketone labeling (7).

E. coli LplA and biotin ligase are mechanistically related, and theirnatural acceptor proteins share some structural and sequence overlap(20). However, the engineered LAP and AP sequences are dissimilar, asare the azide 7 and biotin structures. To test the orthogonality ofthese two labeling methods, we prepared separate dishes of HEK cellsexpressing LAP-LDLR (with a GFP tag to serve as a transfection marker),or AP-EGFR [AP fused to the extracellular N-terminus of the EGF receptor(16)] together with a CFP transfection marker. After 16-24 hours ofexpression, the cells were re-plated together in a single dish. Weperformed labeling by first adding a mixture of LplA, BirA, azide 7,biotin, and ATP to the cells. Thereafter, OCT-Cy3 was added toderivatize the azide, and streptavidin was added to detect the biotin.Results demonstrated that cells expressing LAP-LDLR were selectivelylabeled with Cy3, while cells expressing AP-EGFR were selectivelylabeled with streptavidin. The same results were obtained using LAP-LDLRin combination with an AP-tagged receptor for ephrinA3 (AP-EphA3). Thus,simultaneous labeling of cells with LplA and BirA is possible, withoutsacrificing the extremely high specificity of each system.

We then used this two-color labeling protocol to image LAP— and AP-fusedreceptors co-expressed within the same cell. EGF receptor and EphA3 areboth known to function in cell migration (21,22), and thus we performedimaging on cells migrating toward an artificial wound. HEK cells wereco-transfected with either LAP-LDLR and AP-EGFR, or LAP-LDLR andAP-EphA3. After 16-24 hours of expression, the confluent cells werewounded with a pipet tip. We allowed the wound to partially close over12-18 hours, and then performed s simultaneous labeling with Cy3, andAlexaFluor488 conjugated to monovalent streptavidin (8). HEK cellsco-expressing a LAP-LDLR fusion and either AP-EGFR or AP-EphA3 werelabeled during wound healing by first treating with LplA, BirA, azide 7,and biotin, followed by OCT-Cy3 to derivatize the azide, followed bymonovalent streptavidin-AlexaFluor488 (8) to detect the biotin. The Cy3results showed the non-polarized distribution of surface LAP-LDLR. TheAlexaFluor488 results showed the polarized distribution of AP-EGFR andAP-EphA3 at the wound edge. CFP is a transfection marker. The resultsshowed that Cy3-labeled LDLR was evenly distributed on the surface ofthe HEK cells, whereas AlexaFluor488-labeled EGFR and EphA3 were bothasymmetrically concentrated at the leading edge of the polarized cells.The same patterns were also observed when the LAP and AP tags wereswapped (AP-LDLR and LAP-EGFR), suggesting that the localizationpatterns do not reflect artifacts of AP and LAP labeling.

While the polarization of AP-EGFR to the leading edge of migrating cellswas expected, and has previously been observed using antibody detection(23), the pattern of AP-EphA3 staining is surprising. Previous work hasshown that trans interactions between EphA3 and ephrin ligand expressedon the surface of contacting cells play a role in developmental cellmigration (24) and tumor invasion (25). However, it is unclear thatun-liganded EphA3 should function in migratory processes. Ourobservation of EphA3 accumulation at the free, leading edge of polarizedcells suggests that unactivated EphA3 may play a role in cell signaling,or that EphA3 may be constitutively linked to the actin cytoskeleton.

In summary, we have developed new methodology for labeling cell surfaceproteins fused to a 22-amino acid recognition sequence for E. coli LplA.Small, non-crosslinking probes such as Cy3, AlexaFluor, and biotin canbe site-specifically and covalently conjugated to the LAP peptide in aslittle as 20 minutes. An important feature of our methodology is itsgenerality; any cell surface protein in any cell type can be labeledwith any chemical moiety that can be functionalized with a cyclooctyne.

Many new protein labeling methods have been developed in recent years(9), and a survey of these techniques reveals that a general trade-offexists between labeling specificity and tag size. Protein-based tags,such as SNAP/AGT (26) generally give higher labeling specificity thanpeptide tags, such as FlAsH (27). However, protein tags have greaterpotential to interfere with protein folding, trafficking, and activity,as GFP often does (28,29). We and others [for example, ACP/PCP labelingmethodology (30)] have tried to bridge the requirements of small tagsize and high labeling specificity, by making use of enzyme ligases. Bycapitalizing on the intrinsic sequence specificity of enzymes such asbiotin ligase and LplA, highly specific probe conjugation can beachieved, without sacrificing the small size of the directing tag.

In previous work with BirA, we found that a ketone isostere of biotincould be accepted (16), but not compounds with more dissimilarstructures, such as alkyne and azide derivatives of biotin, due to thestructural requirements of the biotin binding pocket. In contrast, LplAexhibits much more relaxed specificity for its small molecule substrate,while maintaining extremely high specificity for its protein or peptidesubstrate (5). This property allowed us to harness LplA for unnaturalligation reactions in this study. Important next challenges will be toextend this methodology to labeling of intracellular protein targets andto re-engineer LplA for one-step-ligation of fluorophore orphotoaffinity probes.

We also used LplA in combination with biotin ligase to image twodifferent receptors in the same cell. Many problems in receptor biologywould benefit from simultaneous imaging of two or more differentproteins in the same living cell, instead of separate experimentsinvolving one-color labeling of each receptor. The combination of LplAand BirA tagging, which can be performed simultaneously due to theorthogonality of the labeling reaction components, will provide accessto such experiments.

REFERENCES FOR EXAMPLE 2 AND FIGS. 5-6

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EQUIVALENTS

It should be understood that the preceding is merely a detaileddescription of certain embodiments. It therefore should be apparent tothose of ordinary skill in the art that various modifications andequivalents can be made without departing from the spirit and scope ofthe invention, and with no more than routine experimentation. It isintended to encompass all such modifications and equivalents within thescope of the appended claims.

All references, patents and patent applications that are recited in thisapplication are incorporated by reference herein in their entirety.

1-117. (canceled)
 118. A method for preparing a protein conjugate, themethod comprising: contacting a fusion protein with a lipoic acid analogin the presence of a lipoic acid ligase polypeptide to produce a proteinconjugate in which the lipoic acid analog is linked to the fusionprotein, wherein: the lipoic acid analog is a substrate of the lipoicacid ligase polypeptide and comprises either a functional group handleor a directly detectable moiety, and the fusion protein comprises thetarget protein and an acceptor polypeptide.
 119. The method of claim118, wherein the acceptor polypeptide comprises the amino acid sequenceXaa₁ Xaa₂ Xaa₃ Xaa₄ Xaa₅ Xaa₆ Xaa₇ Xaa₈ Xaa₉ Lys Xaa₁₀ Xaa₁₁ Xaa₁₂ Xaa₁₃Xaa₁₄ or a fragment thereof having at least 12 amino acid residues, eachof Xaa₆ and Xaa₁₄, independently, being a hydrophobic amino acidresidue, and each of Xaa₁, Xaa₂, Xaa₃, Xaa₄, Xaa₅, Xaa₇, Xaa₈, Xaa₉,Xaa₁₀, Xaa₁₁, Xaa₁₂, and Xaa₁₃, independently, being any amino acidresidue.
 120. The method of claim 118, wherein the lipoic acid ligasepolypeptide comprises an amino acid sequence of a wild-type lipoic acidligase.
 121. The method of claim 118, wherein the lipoid acid ligasepolypeptide is a functional mutant of a wild-type lipoic acid ligase,the mutant comprising an amino acid residue substitution at the positioncorresponding to W37 in SEQ ID NO:11.
 122. The method of claim 118,wherein the lipoic acid analog comprises a functional group handle. 123.The method of claim 122, wherein the functional group handle is an azidemoiety, an alkyne moiety, or an alkyl halide moiety.
 124. The method ofclaim 123, further comprising contacting the protein conjugate with adetectable label to allow linkage of the detectable label to the lipoicacid analog in the protein conjugate via reaction with the functionalgroup handle.
 125. The method of claim 124, wherein the detectable labelcomprises a cyclooctyne moiety and a moiety selected from the groupconsisting of a fluorescent moiety, a chemiluminescent moiety, aphosphorescent moiety, or a radioisotope.
 126. The method of claim 118,wherein the lipoic acid analog comprises a directly detectable moiety.127. The method of claim 126, wherein the directly detectable moiety isan aryl azide moiety, a diazirine moiety, a benzophenone moiety, achloroalkane moiety, or a fluorophore moiety.
 128. The method of claim127, wherein the fluorophore moiety is coumarin, resofurin, or anxanthene-type fluorophore.
 129. The method of claim 119, wherein Xaa₇and Xaa₉ are E and D, respectively.
 130. The method of claim 129,wherein the lipoic acid analog comprises a functional group handle. 131.The method of claim 130, wherein the functional group handle is an azidemoiety, an alkyne moiety, or an alkyl halide moiety.
 132. The method ofclaim 131, further comprising contacting the protein conjugate with adetectable label to allow linkage of the detectable label to the lipoicacid analog in the protein conjugate via reaction with the functionalgroup handle.
 133. The method of claim 132, wherein the detectable labelcomprises a cyclooctyne moiety and a moiety selected from the groupconsisting of a fluorescent moiety, a chemiluminescent moiety, aphosphorescent moiety, or a radioisotope.
 134. The method of claim 129,wherein the lipoic acid analog comprises a directly detectable moiety.135. The method of claim 134, wherein the directly detectable moiety isan aryl azide moiety, a diazirine moiety, a benzophenone moiety, achloroalkane moiety, or a fluorophore moiety.
 136. The method of claim135, wherein the fluorophore moiety is coumarin, resofurin, or anxanthene-type fluorophore.